People can’t “copyright” their images or pictures of themselves (unless they took it themselves, or got the creator to assign the copyright to them). However, if you copy a picture of Bill Gates as part of your T-shirt, you are likely infringing the copyright of the owner of the original photo.
So to be safe, you may want to take the pic of Gates yourself. Good luck on setting that up…
Right of publicity generally says you have the right to control how and when your image is used to promote products. A fairly recent famous case was one where a company (I can’t remember which one, I think it was an Asian electronics company) did an ad that showed a metal robot, apparently female, posed in front of a series of blocks of letters, some of them covered, others uncovered.
Vanna White sues, saying it violates her right of publicity. And, remarkably, she won!
Even more out there is a car commercial that had a Bette Midler “sound-alike” singing a tune commonly associated with Bette. She sues – claiming that public associated the voice and song combination with her, and therefore thought she had endorsed the car, which she hadn’t. She won.
So, in summary, you mess with rich and powerful celebrities at your peril…