Watch out fuckboy cause I'm gonna prove I'm autistic without even posting an image:
I genuinely don't know if you're talking about autist in the epic 4chan insult sense or if you're referring to people on the spectrum.
I could just as well post generic anime reaction image number 450 and anyone could tell I'm autistic, but I believe genuine cases of people being on the spectrum has much more to do with how they write rather than what they post.
A lot of autists such as myself like writing in detail and articulating themselves as well as possible. If I'm arguing with some guy I like to list realistic counterarguments (no strawmen unless the guy I'm arguing with it a retard to begin with) before he even writes them out because I like to cover as much ground in one reply as possible.
I could keep going but you get it by now.