The dimensions of your graphic, screen resolution and the device displaying the advert all affect the final display
For the best effect you should create an image that is either:
or
These are illustrated to the right, with the shaded area indicating the image area.
The focal point of the image should be position so that it sits within the dashed area in the center of each template. This area is approximately 200px by 200px
This will ensure that the the most interesting part of the image is displayed when the advert is displayed on different devices and screen resolutions.