Art is my hobby. I've always had a knack for drawing/painting in particular, although I haven't developed my skills to the extent I have with my writing (which I am pursuing professionally). But lately I've been thinking I want to start really practicing and developing my art, as I have lofty aspirations of becoming an illustrator on the side--I'd especially love to illustrate some of my own written works.

Of course, everywhere I look I see "don't bother, publishers hire their own illustrators" or "only professional illustrators have a chance of illustrating their own work" type info/advice. My question is, what constitutes a "professional" and how do you get to be that person that publishers hire?

I have been paid for a handful of my artwork, maybe ten or a dozen pieces, commissions that came through casual connections--friends/family members and people who hired me based on recommendations from friends/family members. But it would seem to me that, despite these being paying jobs, they aren't "credits," not the way selling a short story to a pro-paying market is a "credit." So how does it really work? Anyone can put up an online portfolio and say they take commissions, but will that really get you anywhere? Or do you need to be professionally "published" as an artist the same way you do as a writer?

Any advice? Insights? Recommendations for reading material? I understand it's another hard industry to break into and it's going to take a lot of time and perseverance to build up any sort of reputation, I just don't know where to start--other than, say, submitting art to some of the magazines I submit my fiction to.