Honestly, I think you are past the edge for fair use. For starters, your header does not fall under fair use, because you are using trademarked/copyrighted images to sell video games or products not affiliated in any way with the companies whose images you are using. One of the key factors in fair use is that the image is being used for descriptive purposes only - the images in your header (which incidentally also do not seem to be of reduced resolution) do not describe your products.

Another issue I see on your site is outright trademark infringement. You are selling Flash "ports" of commercial games that are claiming to be the original game, using similar graphics and the same names as the original game. When ports of popular computer and console games were created for use on PDAs, the companies sued and won, requiring that the games be substantially different from the original game. The games must have their own name, and be distinguishable from the original intellectual property.

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The best way to learn anything, is to question everything.