I took over an open studios site 10+ Years ago. The original web developer/ graphic designer asked for square thumbnails. then proceeded to crop them down about 30% before displaying on certain pages. Artists reacted by only uploading usable images in the remaining area (so the whole image filled the thumbnail). The uploaded images had the right and tops blank.

Sometimes it’s best to let the originator of the image control it.

Though it would be fun to try some automation..