3. Resizing Existing Images
If you have already images uploaded to your WordPress media library follow the steps below to bulk-resize them.
1. To resize existing images, you'll need to install the Regenerate Thumbnails in order to process images in batch. To do so, in your WordPress admin, go the Plugins menu → Add New.
2. In the search box, type regenerate thumbnails, and click the button "Install Now", then "Activate".
3. Once the plugin is activated, go the Tools menu → Regenerate Thumbnails.
4. Finally, click the button "Regenerate thumbnails for all attachments" to start processing your images.
To view resized images, go to your media library under Media → Library, then apply the filter "Smart Resize: Processed".
Yes, you can regenerate thumbnails for a single image by clicking the link "Regenerate Thumbnails" in your media library.