Similar Questions for How Does Reddit Work from Yahoo AnswersQuestionAnswer
Reddit is a website/online community that happens to create and propogate a lot of memes. The robot, I believe, is their logo.
Try saving it and posting it to Imgur.com, then at the bottom there's a little reddit alien, click that, and it should let you title it choose a subreddit, ect. This is the easiest way I know of... look forward to seeing your comic :D
Honestly, you never can. The name stays in the database to keep links active, and you will have to choose a new username.