I don't really think that avatar pics will be the amazing thing that will Make HLP Great Again.
As it stands, that left portion is already cluttered, let alone with crazy anime stuff or just sarcastic Nicholson pics which add nothing but mild confusion to the tone of the conversation - yes, confusion. More than not, in all these forums, the avatar pics are an attempt by the forum user to cast a tone, a character to their words, but because we're often self unaware, they are not that accurate at all, and there's this continuous dissonance between the image and the words that follow it (child anime avatars followed by long tl dr articulate analytical almost sicentific jargon, for instance, or badass sensible Optimus Prime followed by childish temper tantrum words...).
There's a reason why "Titles" are not self-inflicted, but chosen by others. (that reason is also why I have never asked for one btw, I'm perhaps too self-aware to know better than that).
So the correct question isn't "Why not?", but rather (and I agree with Goober here), "Why at all?". What does it add that we don't already have? Pictorial-Textual dissonance? As things are right now, if you really want to know someone's character, you actually have to do the work
and read the damn textual contribution of that commenter. I like that state of affairs, it doesn't go the route of the lowest possible common denominator.
My 2 cents, of course, and always.
The clutter issue is in people's heads.
Yes, and this problem is often called "design". Whose best mantra ever devised is, and always will be, "Less is More".