For security reasons I beleive.
Create a folder called: image.jpg
Inside that folder, create a PHP file (index.php) place your dynamic image code into that index.php file, and then link to the folder as if it was an image () walla, you can post dynamic PHP images.
But we are not allowed to post images in sigs, and I agree with that
EDIT: Oh, the security reasons would be that you could collect the IP, Browser, OS and views of each person that see the dynamic image.
Well, we haven't even allowed animated avatars for years since some abused that (not related to security, it just got annoying).