This post talks about how you can fix WordPress Image thumbnails not being the right size for your theme.
Why this issue occurs
Whenever you upload an post image the thumbnail is created on upload. This means that the image thumbnail does not need to be created on the fly.
The issue with this is that when you change WordPress Theme the new image thumbnail size the theme uses might be different. If this is the case the theme may be unable to properly size the images.
This can be quite obvious when you start adding new posts as the thumbnails will be cropped to fit the new theme and you will have a mix of old and new thumbnail sizes.
Regenerating Thumbnails at the correct size
To resolve this you will need to re-process the thumbnails again. Doing this will regenerate the thumbnails at the new size and they will be displayed correctly.
To fix this you can use the
Regenerate Thumbnails Plugin.
When you change WordPress themes or change the sizes of your thumbnails at
Settings → Media, images that you have previously uploaded to you media library will be missing thumbnail files for those new image sizes. This tool will allow you to create those missing thumbnail files for all images.