Don't beat around the bush. I've tried to hint that I liked a girl with art about a billion times and they either don't get it or they might suspect it but don't come to the conclusion that it's indeed about them. I mean, I'd feel pretty arrogant to assume a song or piece of artwork was about me, so why wouldn't a woman?
Just tell her man. Don't do it in some grand romantic way by candle light with music playing or any of that romantic comedy bs. Just be an a mature, confident adult and tell her, preferably face to face but by phone works well too, that you really like her and you'd love to get to know her better. You don't have to confess your longstanding love for her or anything.
I know this advice sounds run of the mill, but it really is the best thing to do. Unless you're about to go on a Hollywood style, whirlwind adventure through time and space with this girl, it's not necessary to make a big scene out of it. She probably already thinks you're incredibly sweet for drawing her the portrait.
The one thing I'd be careful of is asking her too soon after she broke up with her boyfriend. I, personally, and I know woman who feel the same way, couldn't give two sharts about dating someone immediately after a break-up. However, some people need time to heal and in some cases may harbor a little resentment towards your sex
.. so giving it a little bit may be of benefit so it's not too awkward.
Good luck man.