I'd say due to the prevalence of trolling on the web, there are definitely some heuristics people use to quickly recognize spam/trolls. The more you look like a spam/troll account inadvertently, the worse I'd say.

Chances are, if you leave your avatar as whatever the service your using's default is, I probably am not going to want to talk to you. Usually, if you're not going to take the 5 seconds necessary to project a personality, you're not going to project any personality in the rest of our interactions.

I hardly every set profile pictures. A profile picture is one snapshot in time of what is probably a very complex being. And what I chose to say in that moment by that one picture may not reflect anything I am or want to convey in the future. I find it hard to decide, and I end up with a picture that says nothing but "here is what my expressionless face looks like". And then how often am I really going to repeat the decision process? My Google profile picture is from 12 years and 2 jobs ago. Although I guess my face hasn't changed much since then.

I'm overthinking it.

People's biases are in full force based on profile pictures. Even when we're aware of those biases and believe we control them. It's very well documented. This fantastic blog post by Julia Enthoven is my favorite on the topic, because it's a first person testimonial about the immediate, measurable effect of simply changing her profile photo at work.

Interesting Topic! If you have a portrait of some kind then the comment looks like a speech-bubble to me. The message can be perceived differently, based on the facial expression on the avatar. That's why I try to have as little "attitude" in my profile-pics. I used to make a new one each year, but somehow I settled on this one since 2016.

I always appreciate avatars in social media, it's some of the closest to character you can get that's projected beyond their diction. Depending on the avatar you can tell if someone is a troll or not, or what interests they have.

Like I see your avatar, and I know you probably like quality anime, and you might be fellow weeb (believe it) 🤘

It helps when someone has a very specific avatar of imagery that represents a group that I don't want to interact with, or I know I'll have more friction communicating with. It also helps to diminish the power of their message if they decide to spew negativity.

I think everybody responds to human faces. Whether I'm aware of it at the moment or not, profile pictures with faces make me feel like I'm talking to another person more, even if the face is just a drawing.

With your profile picture for example, I know it's Naruto, but I mostly just see a smiling face and it makes me feel like I'm talking to a friendly real person.

It's not logical, but I think faces and facial expressions are kind of hard-wired into our brain and trigger emotional responses without even realizing it.

Of course people will judge from your choice of picture - or your choice to not have one.
One of the dangers of services like Gravatar is that you sign up for something using one of your non-important email addresses and don't notice that it's the one you set as a picture of yourself dressed as a pigeon five years ago until you hit send on that important message to a potential new employer...