NextGEN Gallery Optimizer

Download NextGEN Gallery Optimizer to your computer
Log into your WordPress admin
Click Plugins --> Add New
Click Upload (in the header)
Search your computer for nextgen-gallery-optimizer.zip
Click Install Now
Click Activate Plugin
Click Settings --> NextGEN Optimizer to complete installation.

Note: If your computer automatically unzips your download, you'll need to re-compress it first.
On Mac OSX: Ctrl+click the plugin folder --> Compress.
On Windows: Right-click the plugin folder --> Send To --> Compressed (zipped) Folder.

Firstly, you'll need a copy of NextGEN Gallery...
http://wordpress.org/extend/plugins/nextgen-gallery/

Once you've activated NextGEN Gallery on your plugins page, look for a "Gallery" tab at the bottom of your left sidebar in your WordPress admin.

Go to Gallery --> Add Gallery/Images
Click the "Add New Gallery" tab, enter a name for your gallery and click the "Add gallery" button.
Click the "Upload Images" tab, click the "Select Files" button and choose which images you want to upload from your computer, select the gallery you just created from the "in to" dropdown menu, and click the "Upload images" button to begin the transfer.
Finally, to insert your gallery, just create a new post or page (Posts --> Add New or Pages --> Add New), type the shortcode [nggallery id=1] into the editor, and click the "Publish" button on the right to save your changes. Note: The "1" in the shortcode should be the id number of the gallery you wish to display.
That's it! Navigate to the page you just created and your gallery should be live!

If you'd like to optimize this for speed, and add the Fancybox lightbox effect when you click on your gallery thumbnails, simply activate NextGEN Gallery Optimizer from your plugins page. With Optimizer enabled, your site will feel lighter and faster as it ensures the required scripts and styles only load on pages with galleries.
General

Does Optimizer have any known limitations?
Please see the following page in Optimizer's documentation...
http://www.nextgengalleryoptimizer.com/docs/limitations/
How is NextGEN Legacy (v1.6.2 to v1.9.13) faring these days without updates?

As of WordPress v4.2 (possibly earlier) the Sort Gallery page in the admin fails to save changes to the image sort order. It gives the following notice on-save when WP_DEBUG is active: "Notice: Undefined variable: sortArray in ...nextgen-gallery/admin/manage-sort.php on line 19". One workaround would be to simply upload your images in the order you want them displayed.
There are also a few new notices (when WP_DEBUG is active) in WordPress v4.3, however the widgets do still appear to be working: "Notice: The called constructor method for WP_Widget is deprecated since version 4.3.0! Use __construct() instead. in .../wp-includes/functions.php on line 3457".

Why aren't my image uploads working in NextGEN 2.0.xx?
There are many possibilities (typically ownership/permissions issues), but here's an odd one from personal experience. Hopefully it'll help...

I had created several development installs across numerous WordPress releases to test Optimizer with the latest NextGEN 2.0.xx version (v2.0.66.37 specifically), but NextGEN's image uploads weren't working on any of them. All file ownership and permissions were correct, but NextGEN's uploads page was just showing a yellow error triangle with a tooltip of "HTTP error".

After watching the JavaScript console in Chrome, I discovered there was not only a JS error on every NextGEN settings page, but also a "Not found" error on the image upload page itself. It turned out this was due to a compatibility issue with WordPress' default permalink structure (e.g. "?p=123"), as "index.php" was being inserted after the base URL, causing a 404. Changing the permalink settings in WordPress to the "Month and name" structure (e.g. "/2015/02/sample-post/") fixed the issue. Note: this was happening in both WordPress 3.7 and the latest v4.1.
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Wait...my galleries are displaying properly, but my images load in a new window!
If you're sure Fancybox is activated on Optimizer's settings page, and the lightbox still won't display using the "Use WordPress jQuery [greater compatibility]" setting, it is highly likely you have a conflict caused by two or more instances of jQuery loading on the same page. This is typically due to jQuery calls having been hard-coded into your theme.

Please inspect your theme files (the header.php and footer.php in particular) for lines including jquery.js or jquery.min.js and either comment them out, or remove them altogether.

Additionally, make sure you have wp_head(); in your theme's header.php file. It will usually be right before the closing </ head > tag, and is an essential hook for plugins such as Optimizer to be able to load the necessary scripts and styles.
Help!...I can't see the NextGEN "Gallery" tab!!!

Please make sure the "NextGEN Gallery" plugin is both installed and activated on your plugins page in the admin (it's a separate plugin available here). Once activated, the "Gallery" options tab should appear near the bottom of your left sidebar in the admin.
If you can't find the tab for your plugins page either, it's likely you're logged in as a user without administrative privileges. You can verify this by visiting http://www.yoursite.com/wp-admin/plugins.php.

How do I set the Shutter or Thickbox effect?
Firstly, you'll need to deactivate Fancybox on the Optimizer settings page and click "Save Options". After that, simply navigate to Gallery --> Options --> Effects, select your effect and click "Save Changes".
Is this plugin compatible with minification/caching tools?
Yes. However the small, already minified Fancybox script must be excluded from combining/minification or it won't function. This is true of any lightbox script.

For WP Minify, simply add /wp-content/plugins/nextgen-gallery-optimizer/fancybox/jquery.fancybox-1.3.4.pack.js in its js file exclusion options and clear the cache.

For W3 Total Cache, you'll need to select the "manual" minify option at Performance --> General Settings. After that, jump to Performance --> Minify where you can hand-pick the scripts and styles you want minified/combined (excluding Fancybox).
What version of NextGEN Gallery is this plugin compatible with?
Any version since 1.6.2
Premium Version

Why won't my images click-through to a gallery on my [album id=x] shortcodes?
It looks as though there may be a bug with one of NextGEN's settings. Go to Gallery --> Options --> Gallery Settings and make sure the first option "Deactivate gallery page link" is checked (as it is by default).
I've just added a gallery to an album in NextGEN and I get a "Notice: Undefined property: stdClass::$gallery_ids" error message on my page.
The drag-and-drop "Manage Albums" page in NextGEN does not auto-save like the WordPress widgets page. Click the "Update" button and your albums will display as they should.
Help! My slideshows aren't working...they just show a rotating loading circle.
Please go to GALLERY -> MANAGE GALLERY and select the gallery that's causing you trouble. Inside this gallery, ensure ALL image thumbnails are displaying and re-upload them if necessary. If they're missing, the "Path" field may have been changed, which will cause the slideshow to break.

Also, slideshows require two or more images in your galleries to function, else they'll break as above.
How do I get regular pagination on NextGEN Album pages?
By default, Album pages don't include pagination...but you can activate this feature by adding a custom field called "ngg_paged_Galleries" on the edit screen for your album page in the WordPress admin. The value of the custom field should be the number of albums you wish to display per page.
Why doesn't Optimizer support NextGEN widgets?
After spending weeks working on integrating this, I've discovered it's not presently possible in WordPress to:

Conditionally load the required scripts in the header AND
ONLY load them if the widget is actually present on the page.

Since both the Fancybox and the NextGEN slideshow scripts must be loaded in the head section of the page so they don't break, and since the whole point of this plugin is to NOT load scripts on every page, I've had to abandon this idea for the time being. If you know of a solution, do let me know and I'll add it in the next version!
Why doesn't Premium support AJAX pagination on gallery pages?
Apart from the [imagebrowser id=x] shortcode, I had hoped to add support for this on regular galleries and album galleries too, but unfortunately NextGEN's implementation to date is just too buggy, inconsistent and restrictive. For example...

You have to use the Shutter effect across your whole site.
It breaks the [Show as slideshow] links when activated.
It causes several error notices per page (visible in debug mode) read: massive error log.
It isn't coded to work at all on album main pages or on most shortcodes (UI inconsistency).

I've tried to iron out some of these issues myself, but to no avail. Hopefully the team at Photocrati will look into this for a future release.
When I activate the JW Image Rotator in NextGEN Gallery, my slideshows only display a big black box!
Before you can use JW Image Rotator, you'll need to download it here, upload it to your wp-content/uploads folder and enter the FULL filepath to its imagerotator.swf file at Gallery --> Options --> Slideshow --> "Path to the Imagerotator (URL)".

Eg. http://sitename.com/wp-content/uploads/imagerotator/imagerotator.swf

Click "Save Changes" when you're done and you'll be all up and running.

UPDATE: JW Image Rotator now comes bundled with NextGEN Gallery v1.9.9 and above, and requires no manual installation.
Why isn't Optimizer Premium detecting my Album gallery pages or the [Show as slideshow] links?
This issue is likely caused by the "Activate permalinks" option at Gallery --> Options --> General Options, which changes NextGEN's URL structure for things such as the "Show as slideshow" and "Show picture list" links, as well as navigation through your Album pages.

Optimizer Premium looks for NextGEN's default URL structure, which appends query string parameters to much of the navigation. With "Activate permalinks" switched on, these parameters are no longer present, which results in the plugin not recognizing where to add the necessary scripts for either Fancybox or slideshow functionality.

I'm currently working on a way to target these permalink URLs for a future version of the plugin, but for now, correcting the issue is as simple as navigating to Gallery --> Options --> General Options, switching off the "Activate permalinks" setting and clicking "Save Changes".