Of the plugins I am willing to install, one of them is the Force Regenerate Thumbnails.
If a theme is written properly, it will have image sizing code, that will scale an image for every size when an image is uploaded. When these sizes are added to or removed, it’s handy to have all the images get their various versions regenerated. That’s where this plugin comes in.
To be honest, this is more of a developer’s tool than a content creator’s tool. One main warning: If you have a site with a large amount of images in it’s Media Library, running this plugin will go thru every single one. So if you have a thousand images, it could take a long time to finish going thru them all. Doing so could also put a load on the server that hosts the site and thus slow down the site. If you’re going to run this on a large Media Library, do it when site traffic is slow.