Not so sure about a thumbnail conveying what you’re selling, just look here: http://codecanyon.net/category/all?utf8=%E2%9C%93&sort_by=sales_count&categories=all. The first 4 thumbs convey nothing but the authors brand and that’s completely fine. I went with a similar approach by just attaching my brand to my thumbs but I’m sure it doesn’t hurt to try and squeeze a preview of your item in there somewhere.
Yes, that’s why I said above, it should contain the important information of your item like the category, the name, and your brand identity. Also it depends on what type of item are you selling too. With a coded item like on CC then a quick look via the 80×80 tiny square means nothing much, but to visual products like on GR and VH it does. Just my 2 cents
From general experience, thumbnail do help increase sales.