Let me make a forecast on what will happen if there is a ban on images in signatures. Those who wish to advertise will stop using the signature feature and simply append the graphics to the end of their posts. After you have done it once you have the code and it is painless to keep it in a notepad file and paste it into place. Now however you will face a debate about the use of graphics in main posts which is a much harder area to regulate. Particularly as there are many legitimate reasons why someone might use images, for example seeking comments on a choice of book cover.
The fact is the signature is optional. No one needs to see either images or witty quotations if they do not want to. They have the facility to switch them off. I personally think it is a loss but for those who want to suppress them there is that option.
I also do not like the idea that authors should make their book cover their avatar. Whilst it is another opportunity to publicize, the avatar is supposed to represent me in some way. I am
not my book.
Though some people find the advertising annoying I often find it informative, not only for suggesting other material I might read, but for the insight it gives me into other people's writing experience and background, their personality, artistic tastes and motivations. Sometimes the signature can say considerably more about the person than their posts.
That is not to say signatures should be a free for all. There should be simple guidelines laid down. For example:
no more than two cover images (though other books may be mentioned and linked to). Total signature size should not exceed space taken up by five lines of normal text.
If you do something like that, then the vast majority of authors will comply. The minority who do not clearly don't have any interest in participating in the forums sensibly and can be safely banned with no loss.