Sean, yes, you can. You certainly can - and to prove it,
here's a website I built for a lady in Cumbria who takes commissions for animal portraits, and who makes enough money doing it to pay me a decent rate. Probably not your thing, but it shows what you can do if you focus on a particular area.
To echo what aaronsalls said, you'll need to find some sort of niche or speciality, or you're just another general artist.
Hey, how about this: get friendly with some of the folks on writer's forums - especially the sorts of folks who are giving their stuff away online, a la fictionpress.com - and see if any of them would like illustrations doing! Do a few freebies to get your name out there, and then take commissions. Can't be that hard, eh?
Good luck, mate.
Come and talk to me about a gallery site when you've made a bit of cash.
