How to use EWWW Image Optimizer
Once you have EWWW configured the way you want, what's next? Note: if you have Easy IO enabled and verified, that's it, no more work necessary. But if you DO want to compress your local images also, keep reading.
Automatic optimization on upload
The EWWW Image Optimizer automatically optimizes any images uploaded to your Media Library, so that takes care of any new images. To see how much compression has saved you on your images, look for the View Optimized Images button in the top right corner of the settings page. Alternatively, you can switch the Media Library to list mode and check the Image Optimizer column. But what about images you've already uploaded?
One-click optimization for individual images
When viewing the Media Library in the "list" mode, as mentioned above, you will see links in the Image Optimizer column to optimize/convert/restore an image. Also in the Media Library, we can select multiple images and use the drop-down actions to optimize small batches of images.
It's worth noting that if an image has been optimized before, and you use the Re-optimize or conversion links, it forces the plugin to ignore any previous results, and attempt further compression. The one-click links are fine for small batches of images, but what if you have lots of images that you want to optimize all at once?
Bulk Optimizer
As of version 7.4, EWWW IO has a brand new "bulk optimization" process, which you can access via the Optimize Local Images button. The plugin will offer a summary of how many uploads will be optimized, and once you start the process, it runs in the background. That way, you can get back to work more quickly with no need to keep the page open.
If you click View Optimized Images while images are being optimized, you can see the real-time results, as well as view images that are still in the queue. During the bulk process, EWWW IO searches through the database for all images in the Media Library (stage 1). Then, the plugin will also search for images from your active theme, BuddyPress (and BuddyBoss), Meta Slider and a few other plugin folders (stage 2). It will do an actual file-system search through those pre-configured folders, and any additional folders you configure on the Advanced settings.
Legacy (non-async) mode
You can still find the legacy Bulk Optimizer under the Media menu. This is also where you will end up when you use the bulk actions from the Media Library.
The legacy bulk tool requires that you leave the window open, since it is AJAX driven. That said, if you need to stop for any reason, the Bulk Optimizer does have resume capability. So if it gets interrupted for any reason, you can start back where you left off. It normally skips previously optimized images, but you can check the box to Force Re-optimization of your images.
View Optimized Images
As we've seen, the plugin keeps track of all images that have been optimized and you can view every image that has been optimized directly on the settings page. This allows you to remove individual images from the image table so that they can be re-optimized. This is also where you can check to see if images generated by a particular plugin are being optimized automatically, which takes us to one of the advanced options.
Beyond the Media Library (Folders to Optimize and Scheduled Optimization)
On the Advanced settings (Enable Ludicrous Mode), EWWW IO lets you specify folders to include in the Stage 2 scan. For example, if your theme or a plugin is generating extra sizes of your uploaded images, and is NOT tracking them in the media library database, you can add your wp-content/uploads/ folder here to force the plugin to do a low-level scan of your Media Library.
Some plugins store images outside of the Media Library too, like many of the gallery plugins for WordPress. The first thing you want to do is find out if the images created or uploaded by a plugin are showing up in the Show Optimized Images table. If they DO show up in the table, then you can include the folder that the plugin is using in Folders to optimize so that a bulk optimization will compress your past images using the Stage 2 scanner, but you should remove the setting after the initial bulk optimization is complete.
On the other hand, if the images created by a plugin do NOT show up in the table when you upload them, you should leave the folder in the EWWW IO settings, and enable the Scheduled Optimization option.
Fully Integrated Galleries and Beyond
EWWW IO also has native support for NextGEN gallery and many others. Images uploaded to NextGEN gallery are automatically optimized, and NextGEN has a dedicated Bulk Optimization page.
Beyond all that, the EWWW Image Optimizer extends the built-in WP_Image_Editor class which allows for automatic optimization of images created by any plugin/theme that uses the built-in Wordpress image editing functions. Due to image (re)generation bugs in various plugins, this is not enabled by default, but can be enabled with the EWWW_IMAGE_OPTIMIZER_ENABLE_EDITOR override.
Meta Slider and the WP Retina 2x plugin are a couple examples where this is beneficial, but there are many others using these functions for processing images, and all of those can be auto-optimized when the full WP_Image_Editor integration is enabled.
Happy Optimizing!