This plugin lets you easily create clickable and customizable QR codes for any Android app in the Market. They are context aware (checks user agent).
Log in to WordPress as an administrator, and from the Dashboard, go to Settings > Android Market QR Codes.
In the plugin's settings, enter the full URL to an image and it will be displayed instead of a QR code on Androids. Please note that you naturally won't be able to see this image on computers.