When it comes to displaying a thumbnail for a preview of a full image set in a members area, what type of thumb do you folks think will work best?

In a niche specific site for example uncut cocks, should i just show an uncut cock on the preview thumb or, do you think a full body shot would work better?

Im trying to figure out if i need to have some additional programming done to my custom CMS right now which will allow me to choose a couple of different thumbnails based on how the full image set is categorized, and im wondering if offering preview thumbs based on the niche or, just a good 'full body' showing as much as possible will work best.

What do you folks think will be the better option when it comes to preview thumbs inside a members area?

Regards,

Lee