Hi Lexi,
Did you recently change the theme to our theme? If that is the case, posts before our theme have not been cropped so it shows full theme. Posts after adding our theme, have been cropped for optimization. So you are getting different images.
Once you change the theme, you need to regenerate the thumbnail. Follow instructions here to regenerate thumbnails.
Once you do this, all your images will appear cropped. I know this is not the solution you want but theme will require you to use a square based image to work.
Regards,
Pratik