We present a system-level design and programming method for embedded multiprocessor systems. The aim of the method is to improve the design time and design quality by providing a structured approach for implementing process networks. We use process networks as re-usable and architecture-independent functional specifications. The method facilitates the cost-driven and constraint-driven source code transformation of process networks into architecture-specific implementations in the form of communicating tasks. We apply the method to implement a JPEG decoding process network in software on a set of MIPS processors. We apply three transformations to optimize synchronization rates and data transfers and to exploit data parallelism for this target architecture. We evaluate the impact of the source code transformations and the performance of the resulting implementations in terms of design time, execution time, and code size. The results show that process networks can be implemented quickly and efficiently on embedded multiprocessor systems.