Congratulations, you confused me. I think you mean banners in sigs. When you have 100 posts you can request a banner, before that you can put something else in there. Go to your User CP and click Edit Signiture.
An easy way to find pics is searching for images on either Google or Yahoo. But there's a limit on the sizing you're allowed to have. So be sure not to use anything too big.
Most people have signatures with an image or two. This is common, so there must be some rules regarding signature size:
* Image width should not be more than 500 pixels
* Image height should not be more than 400 pixels
* If you have an image that is 500 x 400 (width times height), you cannot have another image in your signature (also, VERY little text -- 5 lines tops)
* You may have two signature images if the height of both does not exceed 400 pixels in height
* Preferred (but not required) image size: 470 x 150
If your signature is within those guidelines, you should be okay.
NOTE: Nudity is not allowed in signature images.
^^^Thems be the sig rules. So now ya can't claim ignorance.