Christina, there's already a working group here for artists. You may want to post your question there, too.

Professional and Energing Artists

http://www.tbd.com/group/249/view

I represented a studio of commercial illustrators for several years. The studio paid me a percentage of the work billed after payment had been received. Our clients were large advertising agencies, not periodicals.

Percentages for reps are based on what they do for you. In my case, I talked to the art directors and artists, quoted prices, obtained approvals and POs, and made sure the finished art was delivered on deadline. The studio administrator took care of billings and collections, and I received 10%.

Independent reps who also do billing and collections as well as interface with clients and artists, charge as much as 25%.

If you're new and not yet known, showing your work to reps wouldn't be a bad idea, whether you engage one or not. They already have contacts with clients, and have a pretty good idea who buys what and where your medium/style are marketable.

The more flexible you are, the better. For example, if you work only in black-and-white line, you're limited. Reps can advise you on that.

I don't know which market or markets you want to market your illustrations in, but I would ask other illustrators to recommend reps to talk to. It's a nice courtesy to offer to buy lunch for their time.