A bit of variation is probably key, take a look at other people's Big Tit TGPs and you'll see they don't all show just a face or just boobs. Looking at 100 thumbs of a pair of boobs without faces would be a bit dull. Pick the models best feature and crop it.
My top 100 thumbs (by niche average click) are very random, groups, tits, faces, asses... that's why I use SmartThumbs to tell me
Good luck