Read original post by Dani Diaz at the Small And Mighty Blog
One of the things I used to hate to do was to create all the images needed to submit your Apps to the Windows Phone Marketplace. You need at least 3 sizes of your App icon. You need:
Small mobile app icon - 99 pixels x 99 pixels
Large mobile app icon - 173 pixels x 173 pixels
Large PC app icon - 200 pixels x 200 pixels