[This post is by Natascha Bock, a Product Marketing Manager on Android. — Tim Bray]
With the latest Android Market update, our editorial team can use your 1024 x 500 “Featured Image” to promote your app on tablets, phones, and the Web. The image can be used on the home page on all versions of Android Market (Web, tablet and phone), on your product page in the Web and tablet versions, and on current and future top-level Android Market pages on phones.
Creating a Featured Image that will do a great job of highlighting your app requires special design consideration.
Not Really Optional

While while many promotional assets are listed as “optional” for the publishing site, we strongly recommend treating them as required. To start with, a Featured Image is required if your app is to be featured anywhere within Android Market. They’re a good idea anyhow; they enhance your product page, making your game or app more attractive to end-users (and more likely to be considered for featuring by our editorial team).
There’s nothing optional about the size, either; it has to be 1024 x 500 pixels.
Do’s and Dont’s

Your graphic is not an ad, it’s a teaser. It’s a place for bold, creative promotional images.
Vivid background colors work best. Black and white are problems because those are the backgrounds used by the mobile-device and Web versions of Android Market.
Limit Text to your app name and maybe a few additional descriptive words. Anything else will be unreadable on phones, anyhow.
Do: Make the graphic fun
and enticing.
Don't: Create a text-heavy
advertising-style graphic.
Do: Do: Use colors that stand out on
black or white backgrounds.
Don't: Let the graphic fade into
the background.
Do: Promote your brand prominently. Don't: Overload the graphic with details.

Your image has to be designed to scale; it will need to look good both in a full-size Web browser and on a little handset. You can rely on the aspect ratio being constant, but not the size. Here’s a tip: Try resizing your image down to 1 inch in width. If it still looks good and conveys your brand message, you have a winner.
On the Web:

On a tablet:

On a big phone:

On a small phone:
More Dont’s

  • Device imagery is tempting, but becomes dated fast, and may be inappropriate if your user’s device looks entirely different.
  • In-app screenshots are inappropriate because your product page already includes a place for these.
  • Just using your app icon is a failure of imagination. You have more room; put it to good use!
Consider the Context

Given the size of the form factor, the phone is the most challenging channel for your image. Below we have both the “good” and “bad” sample images in that context:
Don’t Forget

A 1024 x 500 Featured Image is required for feature placement consideration. Don't miss out on the opportunity!