I have a general question about buying photos. I am not doing a lot of design work, but occasionally I get a request. If I buy licenses under my name at microstock agencies and use them in the design for the customer, can then my customer use this design or should he buy himself the license and give it to me to work on it?

iStockphoto seems to say that I can buy the license under my name.
Since iStockphoto is not the cheapest especially for Els and contributors only get a small percentage I in general want to buy images elsewhere.
I could not find anything in the legal section at the other agencies, DT and FT. Maybe I read too fast or I have to contact support. However support can take quite a while I guess, so maybe someone of you did that before and has an answer?