Quick guide to using reGenerate Thumbnails Advanced settings
reGenerate Thumbnails Advanced (RTA) plugin comes in handy when you install a new WordPress theme on your site that has different image sizes. While the newly uploaded images will be cropped and resized to fit your new theme, the old thumbnails will remain unchanged. Using RTA you can regenerate the thumbnails for all your existing images.
It is also very useful when you have many not-used-anymore thumbnails and you want to remove them while making sure you have all the needed thumbnails generated.
But it will do much more than this if you tweak its settings. So here's a quick guide on what each setting does.
First, you need to to change the settings of regeneration (if you need it). That's displayed on the right side, or if the screen is small enough, top of the screen.
Register custom image sizes. This helps with regenerating sizes which are not registered with WordPress. If you notice a new plugin is adding sizes not present in the list below (which means the plugin is not properly registering its sizes with WP), you can use this setting to register them with WP and make sure the thumbnail regeneration process will create them too for the old images.
Or you maybe just want a new image size on your site. Just add the size here.
It's important to know that if you register new sizes, you must have active the plugin if you want such sizes to be always generated. In other words, if you deactivate the plugin, the new image sizes will not be created for new uploads anymore.
Use selected thumbnails
Select which thumbnails you want to regenerate. All this option does is to decide which thumbnails are regenerated and which not. If a size you don't check is present, it will be left there (unless you check Delete Unused Thumbnails, which we will explain later).
Please note that the fact that you uncheck the sizes here doesn't mean that it will unregister them in WordPress, because they are registered by the theme or plugins, so they will still be generated for newly added images.
The Keep existing checkbox will make sure that if a specific size is already present, it won't be overwritten when regenerating. This is especially useful if the images were optimized with an image optimization plugin such as ShortPixel Image Optimizer (by the way, because of this feature, reGenerate Thumbnails Advanced is the only thumbnail regeneration plugin that works well with ShortPixel Image Optimizer).
You might want to uncheck Keep existing if for example you watermarked your thumbnails and the originals are not watermarked, and you would like to remove the watermark for the thumbnails too. Or if you have corrupted files after a site copy.
Default JPEG Quality
Set the default quality for JPEGs in WordPress. This will apply also to newly added images. The value 90 is the default in WordPress but you might want to go lower if you want smaller images. You should change it to 95 if you are using ShortPixel Image Optimizer to optimize your images.
Time to regenerate your thumbnails.
You can choose if you want to regenerate all the thumbnails or only the most recent ones (last 6 months, three months, etc.).
Regenerate only Featured Images
This will regenerate the thumbnails only for the images which are set as Featured in a post. All the other images will remain untouched. In many cases, it makes sense to regenerate the thumbnails only for Featured images as these appear in posts lists, grids, etc.
Delete Unused Thumbnails
This option will remove the thumbnails that are not selected on the Use selected thumbnails list. It will also remove remove the thumbnails which are present on disk and in the metadata, but are not registered with WordPress (the registered ones are on the Use selected thumbnails). Good for removing stale thumbnails, but be sure they are not in use.
An example when this option comes in handy: you used a gallery plugin which makes its custom sizes and you stopped using it. The thumbnails remain on disk but they're not used any more, so you would want to get rid of those.
Please also note that there are cases when some plugins use additional thumbnails but they don't declare them to WordPress or not even add them to the metadata of the image. Because of this, it's strongly recommended to make a backup first, then after running the regeneration, check thoroughly the site to see if there aren't any missing images. If you notice missing images, restore from backup.
Delete Leftover Image Metadata
Sometimes your postmeta table continues to keep records for deleted posts or for images that you deleted from disk. This option will help remove this metadata thus cleaning up your database. For this option too, it's strongly recommended to make a backup first.