I agree with Linda. People are used to the idea of previous/next buttons or arrows above the big picture. Making the pic itself the previous (& no next) is odd behavior. Frequent users *could* get used to that, but the absense of a way to go back to the page you came from will annoy them and drive them away.

Make it easy on your visitors and put nice clean arrows or something above/below/near your pics. With the invalid one grayed out at each end of the range, it will be obvious what to do.