This is something that should have been in place back when the Marketplace was launched last year, but now when you check out the Gamerpics
or [http://www.xbox.com/en-US/live/marketplace/themes.htm]Themes[/url] on the XBox.com site they actually show you pictures of each screen/pic that you will be buying.
I'm not sure if this was launched at the same time as their Fall Update a couple weeks ago or what, but it's about damn time! How could they expect people to spend money on visual enhancements when they couldn't even preview them?
EDIT: Hmm, it seems as if there are still some of them that aren't shown on there even though you can buy it when online on the 360 (Just Cause is one that I noticed).