Hello Jennifer,
Well, A VM or visitor message is just a way of posting something to someone, that isn't really private enough to be a PM, or public enough to be in a thread. A bit of a lame definition, but that's how I see and treat it. Usually it's just used for conversing or in my case as a mod, a place to receive alerts/questions and requests from other members.
The reason we don't have a sticky on this is because the site is (in shâ' Allâh) undergoing changes, so such a sticky might become redundant in future.
Using avatars with living things is well, questionable. There's nothing wrong with plants, trees and flowers whether they are photographic or artistic for lack of better words. The confusion would arise with imagery of humans or animals. With regards to animated/drawn pictures, then if a face is visible, we don't allow it although you correctly pointed out that some have violated this rule.