I’d been needing a plugin that would fix my past thumbnails that I had already saved. I should have known that there would be a plugin that would do it that’s appropriately named Regenerate Thumbnails. If you’ve ever had to play around with Thumbnail size, then you can understand how great this plugin is.
Full Disclosure: I haven’t used it yet, but it’s from a popular WordPress developer and I found it from another WP core developer who used it successfully. In fact, this post is so I can easily find this plugin in the future as much as anything else.