I happen to be the owner of such a company
Realms of Evil Games, specializing in terrorizing the RPG community with books of villainous evil and scum.
Most RPG companies pay on a per illustration basis...I have never heard of any of them giving royalities of books sold.
I posted an ad for artists in the Looking for Work forum I believe, if you're interested in helping RoE Games out, you can look there. We may not be able to pay you much (as we are small and just starting), but you have to get your foot in the door somewhere right?
Deadlines: That depends on when you are hired actually. If you walk into a job when it's half over, you can expect the deadline to be shorter than normal. However, many companies plan to put out books several years in advance and take that amount of time to write, edit, and illustrate their books before publishing. Deadlines are usually pretty long and many artists work for more than one company at a time to keep busy during that time.
The best place to find RPG companies would be at EN World ( http://www.enworld.org
) as most of the company heads post there themselves. I am sure you could get a job doing something by posting there.