If you are a Windows Phone App developer then head on over to this web based tool and have a play: http://snac.nokia.com/2/pages/marketplacehome
If you are still reading here is the simple walkthrough:
Go to the WindowsPhoneMarketPlace and retrieve a link to the App you want to use
Paste this into the space provided:
(this will trigger the population of the other feeds).
Click on “Generate Banners” to create the full set of banners (see below)