Try to pick an image that's basically square. Don't worry too much about formatting though. As long as you pick a web image (JPEG or GIF), it'll be resized and cropped to a circle.
Selecting an Image File
Click the "Browse..." button in the form window. This will display a list of files on your computer. Navigate to the file you'd like to contribute and click the "Open" button.
Changing Your Photo
Don't worry, you can change your photo at any time. Just select your name from the form menu, and click the "Browse..." button to pick a different file.