Members Area Gallery Preview Thumb Question

Just wondered if those of you who own paysites had noticed if using a specific type of image for the gallery thumbnail made any difference to the usability / popularity of members viewing galleries?

For example, do thumbnails showing a face tend to get clicked on more or, do full body or random thumbnail images work better?

Right now im in the process of selecting thumbnail images to use for our member area gallery previews but before i do all 3k+ of them i want to find out what the best type of thumbnail image to use is.

Anyone every tested this before?

Regards,

Lee