Adbots actually don't post images due to the rule where you can't post images until you've made 15 posts, nor do any other types of spambots, and I honestly don't see people spamming images/videos often, or even at all, and if so they're dealt with. I just think the 15 post rule seems to cover it completely, without being too high for members who only just joined and want to post images but are hindered by the rule, and not too low for adbots to quickly take advantage of it.