What is the "Feature Graphic" for Android apps?

EDIT: Google announced the web version of the Android Market earlier today. It appears that the "feature graphic" is used (in a smaller size) for the top features (see large images near the top: http://market.android.com/) and at the top of individual app pages.

The little I found was this:

• Use a safe frame of 924x400 (50 pixel of safe padding on each side). All the important content of the graphic should be within this safe frame. Pixels outside of this safe frame may be cropped for stylistic purposes.
• If incorporating text, use large font sizes, and keep the graphic simple, as this graphic may be scaled down from its original size.
• This graphic may be displayed alone without the app icon.

Based on the size and name alone, I guess that it's most likely to be used for your app if it is featured and the large size is so that it looks good on the tablets that are coming out.

You can add app icon, app title, tagline, photo that represents the idea behind app, etc... Look at attached images for better idea.

The feature graphic is no longer optional.

See this Google image search for many examples.

android-market-featured-image discusses content do's and don'ts.

It's that flashy image on the top of the (new) market, for when your app is featured there. I didn't bother with it for my app :)