If you feel uncomfortable with it or think you’re not capable of handling it then just reject or recommend someone else for the project.
As to designing a mobile website (especially for iPhone) I doubt that the average iPhone user has images disabled or even knows how to do it. Design wise there’s not that much different than on regular websites, I guess, since mobile Safari just scales down regular websites. You have to optimize the images for best performance (small file size) while still having them look good. One thing you should look into is CSS media queries