You're allowed to post, no worries there

As I'm not an artist, I'm not entirely sure on the best way to apologize, though I do know stuff like that can be a touchy subject.
It's too late now, what's done is done. Concentrate on being really positive around her, when you get chances you could probably compliment her art, etc. That's more if she was quite upset about it, otherwise, you could leave it in the past. Plus, I find even if there are disputes and stuff between friends, it's often forgiven relatively fast and forgotten (at least, that's how it often is with my two close friends).
For her birthday, make sure you let her see that you're excited for her, I guess. If you were already planning on giving her a gift, then you should of course still do so. Saying happy birthday will also help (and you could also remind your other friends to say happy birthday to her, it might help). Just try and be thoughtful and nice, I guess.
I really don't think you'll have to much trouble patching it up, because you do feel guilty about it. And I'm sure you'll find other ways than what I've suggested, too.
Honestly, I don't think there's much more I can say, so hopefully this was somewhat helpful... XP