Any time you change a theme in WordPress, it's a good idea to regenerate your thumbnails. This process creates all the correctly sized images for your theme. Themes use different, predefined image sizes and without the correct sizes your site may look sloppy.
This process does not get rid of your original uploaded image. It simply adds new image sizes to your images folder to be used in your new theme.
* This requires the Regenerate Thumbnails plugin be installed. Make sure to install the Recommended Plugins prior to this step.
So, let's get started:
1. Go to Tools > Regen. Thumbnails
2. Click the Regenerate All Thumbnails button
3. A real-time update screen with progress bar will appear.
If you have a lot of images, this process may take some time DO NOT close this screen until all the images are done updating. You can work in another browser tab while this happens but don't close this screen.
If for some reason, you need to stop the process, press the Abort Resizing Images button. You can continue later.
4. When the progress meter reaches 100% and you receive the All done! message at the top of the page, you are good to go.
The Debugging information will also give you details on each image that was regenerated and any failures.