I've done digital rat commissions before, using Photoshop CS2 or Illustator. It's good because it's fairly fast, and since it's digital you don't have to worry about the cost of replacing your real media. You also dont have to worry about postage, and payment can be done through Paypal so it's quick and easy for everyone
I story boarded and did some preliminary sketches and concept art for a standalone comic about 1920's era gangsters who rode giant rats through the streets of new york, but never ended up completing the project. My avatar is a heavily cropped block from a splash page.
I do memorial plaques and so on, which are more or less portraits. I haven't really recieved rave reviews and orders. I'm working on plaques 2 and 3 now. A second of rats and one of a cat. I do more realistic work, instead of cartoon types. And I also do memorial graphics for sigs but no luck there either. So much for extra funding to my rescue lol
Some people have good luck, others don't. Perhaps you'll be luckier than some lol