Read original post at Dave Bost's Blog
Recently, I contributed an article on the Marketplace Test Kit to Jeff Blankenburg’s 31-days of Mango. The Marketplace Test Kit should be used by all Windows Phone developers to perform a check-point on their app, and it’s requirements, prior to submitting it to the Marketplace. If your app doesn’t pass the test kit, it will certainly fail certification in the Marketplace submission process.
One of the Automated Tests within the test kit is to verify you have the correct sized images and file type requirements for your Marketplace artwork.