Hmm... I've sold photographs at a gallery before and now that I think about it, it was definitely all under the table. I didn't even write receipts. Oops.
I would start a website and form an LLC. Any income you make over a certain amount (varies by state, I think) needs to be reported for tax purposes.
Definitely don't quit your day job, but use this for cash on the side... more like money to support your hobby.
Have shows, collect your best artwork and frame it and have it shown at churches and college campuses and art galleries. Print up posters to your website.
I think the going rate for paintings is this: charge three times the amount it cost you to make it. I could be wrong though. Definitely keep researching all of this. You may not even have to form a company.