you cannot use an image that displays he following: .........
This type of specific rule is prone to users testing their limits. "This isn't a kitten, this is a adolescent cat." Or, "This isn't a kitten, this is a lion cub." Both violate the spirit of a "No kitten pictures" rule, but not the letter.
Instead, I'd focus on something much more generic. If you are concerned about adult images, add in a restriction about being "Safe for work". If you are concerned about flashing, seizure inducing images, add in a requirement that the image must be static.
Size limits are also important. You probably want to do this in terms of both pixel height/width and size on disk. This keeps the amount of screen real estate used under control and prevents users from uploading a file that is gigantic.
When a user violates these policies, step in and remove the violation or give the user a narrow time frame to do so themselves. If the behavior occurs again, restrict their ability to modify the signature or the avatar.
Some forum software allows you to set the policies mentioned above. For example, PHPBB allows an administrator to set the maximum number of characters, number of links, size of images, as well as where certain BBCodes are allowed. Having this technical solution is a great way to keep everything uniform and ensure that everyone meets the most basic of requirements.
It also has the ability to restrict setting on avatars:
These cover the technical side of things, which means that is one less thing you need to worry about as a moderator. Now you can focus on flashy, adult images instead of worrying if this signature image is 5 pixels to wide or 4 KB to large.
