Hi,
I am fairly new also. But I can address a couple of your questions.
"I mean every affiliation program give different way of presenting thumbs for free hosted galleries. So do i need to create by hand a thumbnail which would be the best converting thumb based on the gallery images?"
My site is not a TGP, but I was running into issues where each affiliate offered different aspect ratios for their images. Hence I was unable to get consistency when creating the main teaser image of each gallery. See:
http://www.phoenixerotica.com/photo-galleries
I emailed a few of the affiliates to see if it would be ok to crop one image per gallery (some had specific rules against this), and the handful said it was fine. Even the ones who had strict no logo cropping rules.
So I just ran with it. I guess if any of the others I didn't contact have a problem with it they'll let me know.
Regarding software to crop/resize, most image editing programs will have a batch process option. Photoshop, etc. But if you're looking for something lightweight, I would recommend Irfanview:
http://www.irfanview.com (It's free, and useful for simple tasks like resizing, cropping, etc.)