Here is a tidy list of the supported image ad dimensions for the Google Display Network.
Desktop Browsers
Acceptable file formats: .GIF, .JPG, .JPEG, .PNG, .SWF
File size limits: 150 KB or less for all files.
Supported ad sizes:
250 x 250 square
200 x 200 small square
468 x 60 banner
728 x 90 leaderboard
300 x 250 inline rectangle
336 x 280 large rectangle
120 x 600 skyscraper
160 x 600 wide skyscraper
300 x 600 half-page ad
970 x 90 large leaderboard
High-end Mobile Devices
You can also create image ads to show on iPhones and other high-end mobile devices with full HTML browsers.
Acceptable file formats: .GIF, .JPG, .JPEG, .PNG
File size limits: 150 KB or less for all files.
Supported ad sizes:
320 x 50 mobile leaderboard ***UNIQUE TO MOBILE***
200 x 200 small square
250 x 250 square
300 x 250 inline rectangle
320 x 100 large mobile banner ***UNIQUE TO MOBILE***