That's not the right question, because the only way you'd violate someone's copyright to a celebrity's image is if you copied some photographer's rendition, or some other painter's painting, of that celebrity, in a way that was "substantially similar" to those copyrighted works.
What you should be concerned about is how not to violate the celebrity's exclusive publicity rights in their own image. Basically there are 2 ways.
One is to get the celebrity's permission to paint their image and sell your artwork. I don't think you have much chance of doing that, since celebrities manage their images and their merchandising very carefully.
The other way is to "transform" the image of that celebrity so that your 1st Amendment rights of expression trump the celebrities rights to control the distribution of their image. How do you know when you've tranformed the image enough? Hard to answer question. The CA Supreme Court put it like this: "whether the work in question adds significant creative elements so as to
be transformed into something more than a mere celebrity likeness or imitation." In other words, you have to make the image your own. See their opinion in Comedy III v. Saderup, linked below.
See your own IP lawyer before you launch this business.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.