I'm a commercial artist and you need to focus on the small venues like Dirt Rag, though you might drop Richard Cunningham a line at MountainBikeAction, he is a real staight shooter and I expect he is also very busy. Rodale keeps a pretty tight line on their magazines [I think everything is written by interns, please prove me wrong] and Velo News is being sold. Bike magazine might be a possibiltiy or bikemag.com. Your chances of getting accepted to an online source are easier then a print format. You'll need an artist release notice, most magazines have their own.