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, you can switch the Media Library to list mode and check the Image Optimizer column. But what about images you've already uploaded?
If you are 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?
You can find the Bulk Optimizer under the Media menu. This is also where you will end up when you use the bulk actions from the Media Library. This lets you optimize all your images in one fell swoop.
The first step is to run a Scan where EWWW I.O. searches through the database for all images in the Media Library (Stage 1). In Stage 2 of the scan, the plugin will also search your active theme, BuddyPress, Meta Slider and a few other plugin folders. While Stage 1 relies on the database to tell it what images exist in the uploads folder, Stage 2 will do an actual file-system search through the pre-configured folders, and any additional folders you configure on the Advanced settings. Once the scan is finished, it will let you know how many images have been found before continuing.
Once you start optimizing, you'll need to leave the window open, since the bulk process is AJAX driven. However, 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.
Show Optimized Images
As we've seen, the plugin keeps track of all images that have been optimized. Since EWWW I.O. can optimize images outside the Media Library, you can visit the EWWW IO Tools page (in the Tools menu), click Show Optimized Images, and it will let you browse through ALL optimized images.
From here, you can also remove individual images from the tracking 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, EWWW I.O. 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 add your 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 Optimize will compress your past images using the Stage 2 scanner, but you should remove the setting after running a Bulk Optimize.
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 settings, and enable the Scheduled Optimization option.
Fully Integrated Galleries and Beyond
EWWW IO also has native support for NextGEN gallery, FlaGallery, and NextCellent Gallery. Images uploaded to those galleries are all automatically optimized, and each of these have their own Bulk Optimization pages. 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. Meta Slider and the WP Retina 2x plugin are a couple examples, but there are many others using these functions for processing images, and all of those will be auto-optimized.