Thumbnails that show the whole image

I would like to figure out a way to change the thumbnails on the home page to show a small, complete version of a featured post image without the square cropping that is standard with the theme. The theme seems to override the basic wordpress settings for thumbnails and I’m no code expert, so a cursory browse through the editor and a few trial-and-error attempts at fixing the problem myself have failed. Any help would be greatly appreciated. Thank you!