i haven't found big pics to sell better when used on tgp galleries and freesites - heck, i make good sales with screencaps when i point out they're pics from the actual videos.

i have found that bigger pics can sell better on tours if they're high quality, but better to have sharp clear medium sized pics than blurry or low quality bigger pics.

honestly what sells best for me isn't about size (as long as we're not talking tiny pics) but about how well-chosen the pics are.