Multihop message routing is a well studied and widely documented area of research in wireless sensor networks (WSN), where the dynamic and lossy nature of the wireless medium and severe resource constraints pose major challenges. The vast majority of related research focuses on reliable and power-efficient transfer of small amounts of data, such as low frequency sensor readings or event detections. However, in a few but notable WSN applications, reliable transfer of mass data is essential. The paper describes an efficient multihop bulk transfer service along with a complete sensor network application utilizing it for on-demand image transfers. The paper focuses on the unique problems of the service, such as resource allocation, flow control and mobility throughout the modeling, simulation and implementation phases. Models of the protocol have been built and simulated in a probabilistic wireless network simulator. The prototype implementation targets TinyOS, a well-known WSN operating system.