Do I need to get permission from celebrities before I can post pics of them in my website?

It depends on how you use them. It must be clear that they are just representations of your work and that the celebrities do not endorse you nor recommend your work.

Celebrities have the right to license their names and likenesses and can sue anyone who uses either without express permission. However, if you took the photo or someone did on your behalf and the celebrity was aware that the photo was being taken, you can use the shot on your site to promote your work. Becareful not to imply anything beyond the fact that your gave the celebrity a gift and he/she consented to be photographed holding the gift. You can't say that the celebrity loved your work or that he/she endorses you unless they have voluntarily done so and given you permission to say so. FYI: You should get a release from any person you feature on your site whether they are famous or not.

The celebrities have the right of publicity for their images. So you cannot sell pictures of them or otherwise use pictures of them as advertising to make money. However, there is an exception for fine art. If your art may be considered fine art, particularly the paintings, then it fits the exception. It is best to discuss with a lawyer directly and show them the pictures and the situation and let them make the judgement call.