Legal Question in Intellectual Property in California

What you are describing would be considered "derivative works" in the intellectual-property field, and are within the copyright protection of the original artists. I'm not sure how far you'd need to go in altering the original art for it to be considered your original work instead of derivative, but probably a lot farther than you have in mind (or describe in your question).

Examples of derivative works would include a screenplay produced from a novel; a translation of a textbook into Japanese; and copying bronzes into marble and vice-versa.

Bryan is correct. The real issue though is the copyright protection on the works that you're looking at. Getty Images, for example, charges a license fee based on use, and you should check out their pricing structure, as it may fit within your client's budget. You can also approach the poster of the photograph, tell them what you're doing, and ask for their permission, and I think you'll find many people would be flattered and willing to help. You'll need their written permission. As a Business Attorney, I often draft those types of agreements. Best of luck with your new venture.