The wrong size images maybe break the layout or slow down your blog, so to make sure the Featured images show up correctly and uniform, you need to do this work once right after installing the new theme.
You need to install this plugin first: Regenerate Thumbnails
Go to you Dashboard –> Tool –> Regen. Thumbnail –> Click Regenerate All Thumbnail. Wait to 100% complete.
Note: This tool don’t delete any images on your blog, it create the many copy images in the different sizes, and this tool don’t change/affect the attached images in your posts, you will see the changes only on the Featured image.
