Asylum-- doing original sketches of copyrighted characters isn’t illegal at all. It’s COPYright, so if you do an original painting or illustration and sell it to someone and never copy it, no law has been broken. You could do a whole gallery show of copyrighted characters, and as long as you don’t sell prints of those paintings or illustrations, or a book of reproductions, you’re in the clear. And the reason there aren’t businesses that do it-- well, it’s difficult, repetitive, and there’s not much of a market out there in the real world. Go to a comic convention, however, and you’ll see every artist there doing it, some for money, some not. When I’m sitting at the DC booth I don’t charge for sketches, but mostly because that would be tacky, not because DC would have any legal complaint.
Another major source of income for comic artists is selling original art. If you did work on a popular series, pages can sell for pretty decent rates, particularly if you had the foresight to put popular characters, a lot of different characters, characters using their powers, or characters with no clothes on on the page. Depending on the comic, the page may sell for more or less if the word balloons are actually printed on the page (like, for instance if the comic was written by Neil Gaiman, Alan Moore, or Garth Ennis it would sell for more, and if it was written by Todd McFarlane, less).
Small publishers generally pay a percentage of the future sales of a comic. If you created the comic entirely, a publisher will put up the money to print and distribute it and then after they break even, split the profit 60/40 in your favor. Some larger small publishers (if you follow me) will pay a small page rate upon receiving the art as an advance on that royalty payment.
Other artists make a fairly decent but entirely anonymous living doing storyboards for animated movies, TV shows and commercials. The money is excellent, but no one but art directors ever sees their work, and the deadlines are grueling.
And, Astro, as you may know, in the last 4-5 years, a Florida-based comic company called CrossGen created a system in which artists and writers were salaried, given a set workload, made to relocate to Tampa, and did a passable job of making a comic company into a cubicle farm. It worked okay to not so great, depending on which former employee you speak with. I believe they’re still in business, and some people like their comics… more power to 'em.
Anyway, my 2 cents…
Zander