Creating thumbs is not a problem. I do that every day.
I'm just wondering if there's a gallery program that suits thumb previews (apart from spotbrokers yet to be released program). To work properly it would most likely need the thumbs to be served by them, or they would need some sort of update API since the paid galleries will always be changing.
With text TGPs it doesn't matter, as you are just sending the surfer using a generic niche description. With a thumb preview, you are showing a specific image that they expect to see when they click.