You may be asked to create an ad that is optimised for "retina" or "high pixel-density" screens. The best way to do that is to make sure all of your images are at least 1.5 or 2 times the intended display size. For example, if your display size is 320x240 px, then your image size must be 640x480 px.
IMPORTANT: Please don’t change the ad unit dimensions to support "retina" screens. You can use the same template to target different screen types.