Find out who owns the particular picture you used. If it is in the public domain, there is no problem. If it is copyrighted in the traditional sense (i.e. not released under some sort of special license, such as Creative Commons - though given that those licenses are recent, it's unlikely), then your painting is considered a derivative work, and the copyright owner can legally prevent you from doing anything with it that s/he doesn't like.
If you do find out that that's the case, I think that the best thing to do would be to keep your transaction very quiet, so that the copyright holder does not find out about it. It costs them money to sue you, and if it will cost them more to sue than the profit they expect to receive, they probably won't bother. Key to this is that your case doesn't somehow become widely known - if the rights owners see that other eyes are watching, they will sue just for the publicity alone, to make an example of you.
Your professor is wrong, though (or you understood him/her wrong) - copyright is not just for "claiming someone else's work as your own" - that's called "plagiarism". Copyright is the right to control derivative works, whether or not those derivative works properly attribute the original source.
Here's a similar case that you might be interested in, regarding that famous red & blue Obama image (that it turns out was based off a newswire photograph):
http://en.wikipedia.org/wiki/Barack_Obama_%22HOPE%22_poster#Origin_and_copyright_issueshttp://painting.about.com/b/2009/01/23/fair-use-debate-around-obama-painting.htm