When you first install the theme, the images may be cropped into different sizes. Install plugin – “Regenerate Thumbnail” or “Force Regenerate Thumbnails“. Then go to “Tools” > “Force Regenerate Thumbnails” > press the “Regenerate All Thumbnails” button and wait the process complete. This will fix the inconsistent thumbnails for most users.
STILL DOESN’T WORK
If you’re already regenerated the thumbnails and still aren’t in the correct size, it most likely because your images are too small. Try to upload the images size that is larger than the designated size or at least 1200px X 1200px.