I have a handful of plugins I use on my sites, and I’d like to talk about one in particular I use very often, specially when I am working with or editing a new theme. AJAX Thumbnail Rebuild
Whenever you switch themes, or create your own theme and add or change the size of the post thumbnails (or any other image) it might not display as it should. This is because WordPress didn’t know about your new thumbnail size when it created the thumbnails for your previous posts.
What this plugin does is, it creates your new image sizes for all the images on your site. For example, on my own site, my post thumbnails are 140×140. If I were to use another theme such as Blue Splatter which uses a 645×175 post thumbnail, I wouldn’t have that size image within my media. This plugin would create the 645×175 size so all of my old posts would display the thumbnail properly.
It’s very easy to use, just goto the plugin page in your admin section and run it, once it’s done (takes about 3 minutes for my site) you’ll have your new thumbnail size for each image.