Most WordPress themes these days make use of Post Thumbnails or Featured Images as they are also called - these are one image that you associalte with your blog post which can be used in various sizes around your theme - the size and amount of thumbnails that are set is based on what your theme specifies. If you change to a different theme at a later date you will see that the thumbnail images are distorted - being stretched or compressed depending on the new theme's sizes.
Im in the process of moving a blog to a new theme and needed a way to regenerate the thumbnails for the new theme, I found a free plugin which will automate this process called Regenerate Thumbnails, lets take a look at how to use it.
Install The Plugin
Download the plugin, then in your WordPress admin, visit “Plugins” > “Add New” > “Upload” and upload the file.
To resize featured images indivually, in your WordPress admin visit "Media" > "Library" and you will see all your uploaded images listed - if you hover over them you will see a "Regenerate Thumbnails" link appear :
On this page, you can also tick the checkboxes next to images and then from the "Bulk Actions" drop down box, again choose "Regenerate Thumbnails" :
Finally, to regenerate all thumbnails on your site at once, visit "Tools" > "Regen Thumbnails" and then click the "Regenerate Thumbnails" button :
The plugin will then start processing all the images and resaving them in the new size specified by your theme. This will take a while if you have a lot of posts, it took around 5 minutes for my site with 100 posts but the plugin does display a percentage progress meter :
This is a nice and simple plugin that does exactly what is required - give it a go next time you change themes and need your thumbnails recreated.