I'm sure I'm not the only one who hated the Grid View for Steam library because some games had problems showing the grid icon or that non-Steam icons show the tiny app icon instead of a high quality image.

Well, I have good news. If you go to your steam setting and try the new Steam beta you can right click on a game in Grid view and Set a custom image. This works for both steam games and non-Steam shortcuts.

Here are some pics with my new and fixed grid view

Imgur album

You can now add images to mac version of GTA games, beta version of Team Fortress 2/Dota 2/Counter-Strike Source or replace the grid better pics(like a grid image for Anomaly: Warzone Earth without the review scores and such)

Here are 2 topics where you can look for new grid images for your games:

SPUF thread - nevermind the warning. they removed the NSFW pic
Neogaf thread

BTW: The pics should be 460x215 or higher, but with the same aspect ratio, otherwise you get ugly black stripes.