Maximum amount of images, quotes and text in your signature: As much as you can fit within the other rules.
Maximum signature width: 500px total. If you have a 250px image and a 300px image side by side, that would be violating this rule.
Maximum signature height: 100px total. If you have a 75px image and a 50px image one below the other, that would be violating this rule.
Maximum signature filesize: 50kB total. If you have multiple images then the filesize is added between them. Smilies also count.
Maximum avatar filesize: 20kB.
Restricted file types in signature/avatar: None. You can have it animated if you must, but be sure that it meets the other rules (More importantly filesize).