There are at least 2 potential problems you'll need an IP lawyer to help you with. The 1st is that if you just change the colors of an existing photo, that's not a change that's "transformative" enough to avoid liability for copyright infringement. Whoever took the photo, if they didn't sell it or its copyright, is the owner, and only copyright owners and their authorized licensees have the right to create derivative works from the originals. But if you, for example, morph the images so that you're commenting on the images, like maybe putting Groucho glasses, nose, etc. on Glen Beck's face, that would be transformative enough so that you'd be the creator of a new work.
The second big issue you have to consider is that the subject of the photos, the celebrities themselves, have an exclusive "publicity" right to exploit their own images for profit. Again, you're free to parody and comment on a celebritiy using their image, and you're also free to make new transformative art using their image, but you can't just use an existing image with maybe a color change and put a quote underneath it.
CA has a very strict publicity law that includes disgorgement of profits, punitive damages, and attprney's fees, and if there's a group of people you don't want to pick a legal fight with, it's rich celebrities.
How much change is enough for your additions/subtractions/changes to make a work transformative? It depends, and many clients and lawyers have spent countless hours litigating that question. See one prominent CA Supreme Court case below, and see an IP lawyer before you launch your 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.
You are talking about making something called "derivative works." You take the original work and you alter it in some way. Generally speaking, if the original work is subject to copyright protection, then you can't prepare a derivative work based upon that original work without the consent of the copyright owner.
I'm not licensed to practice law in California so don't take what I say here as legal advice. It's simply my analysis of the facts you present in light of general principles of law. If you need legal advice, please consult a lawyer who holds California licensure.
There's a good chance that you would be infringing the copyrights in the photos (since the images on the shirts would constitute derivative works of the original photos and what you're doing would probably not qualify as a fair use, unless the alterations of the photos and use of the modified photos were completely transformative and the tee-shirt licensing market for the original photos would not be affected). An equally big, if not bigger, problem is that you would likely be violating the celebrities' rights of publicity. The right of publicity gives someone the ability to control the commercial use and exploitation of their name, likeness and other identifying features. Sometimes right of publicity is protected by statute (as I believe it is in California), sometimes by the common-law tort of invasion of privacy, and sometimes by both. You can violate a celebrity's right of publicity even if you own the copyright in the photo -- they are two different IP or proprietary rights. You need to consult with an attorney before going any further with this idea.
The foregoing is intended for informational purposes only and does not constitute legal advice on any specific matter. You should consult with an attorney licensed in your jurisdiction before taking definite action on this or any other legal matter.
Since you mention the Obama face, you should look at what has happened concerning that image: the conceptual artist Sheperd Fairey was sued by the Associated Press for copyright infringement based on the artist's use of AP's photograph. It's become more complicated than that, but the gist is that colorizing an image and then putting it on to shirts for sale is risking trouble, not only under copyright law, but also rights of publicity.