Your questions were answered in the previous posts, but let me just give you some food for thought. Those works, containing celebrities, did you actually buy rights to use and reproduce those photos? Photos like that are usually taken by professionals, and as such, they'd usually stand for their rights on those photos if they ever see your works. There are certain laws regarding what is called "derivative works", but as I'm not sure of the way they go exactly, it's up to you to check that out. Know your right, but also your obligations
Unless you signed a contract giving your rights over the picture to your client, it is already illegal for them to use your work commercially. You retain all copyright to your work, so they can't print it on t-shirts, or let another person use it without your permission, or edit it.
If you sold them a physical painting, though, they may sell the physical painting to another person or paint a mustache on it or destroy it. But they can't copy the image in any way.
So you don't need a contract, the copyright law already protects you, however you should probably inform your clients that you are retaining your rights.