This paper addresses optimizing cache allocation in a distributed image database system over computer networks. We consider progressive image file formats, and `soft' caching strategies, in which each image is allocated a variable amount of cache memory, in an effort to minimize the expected image transmission delay time. A simple and efficient optimization algorithm is proposed, and is generalized to include multiple proxies in a network scenario. With optimality proven, our algorithms are surprisingly simple, and are based on sorting the images according to a special priority index. We also present an adaptive cache allocation/replacement strategy that can be incorporated into web browsers with little computational overhead. Simulation results are presented.