I believe "Thank You" pictures are generally accepted here, as long as they are simply that.
I love getting "Thanks for subscribing" pictures on my wall, but I have also gotten a couple postings of "Check out my comic." Those are not cool.
The Thank You drawings are a very nice, subtle way of advertising your comic (and a great way to discover new ones!), but a post on a random person's wall of a blatant "subscribe to my comic, please" is too much and is just annoying.