I do these thank-you posts to a.) show genuine appreciation for my new subber! and b.) signal to them that it is 100% okay to approach me and talk and comment and whatnot, since some people are a bit hesitant about that.
Here are the images I've used:
... picking the main characters seemed like a good idea, since they are the focus of the story and will be the thing people remember best about it.
I also do milestone-thank yous - whenever I reach a certain number of subscribers, I draw a picture and post it on my wall, and click the little bell on the post to alert all of my subscribers. This is where I take the chance to draw all the other characters in my cast, including the very minor ones that only appeared for one chapter and won't be coming back.