Your signature image may NOT exceed 600x250 pixels in size. The image below is an example of a correctly sized image (though it is 500 pixels wide, not 600)
Your signature may contain up to 5 lines of text (as viewed from a desktop/laptop) OR 3 lines of text and an image (any size adhering to aforementioned rule).
Additionally, the following rules apply to signatures:
You may NOT put offensive images or text in your signature.
You may NOT link to dangerous sites.
Your image may not exceed 600 in Width.
You may not use an image that tracks users' Ip address. They're cool yes, but they freak some people out.