The use of small and miniature unmanned aerial vehicles (UAVs) for remote sensing and surveillance applications has become increasingly popular in the last two decades. The intermittent connectivity in a sparse UAV network makes it challenging to efficiently gather sensed image data. This paper investigates the communication of sensed images from a set of mobile survey UAVs to a static base station through the assistance of a relay UAV. Given the planned routes of survey UAVs, a set of relay waypoints are found for the relay UAV to meet the survey UAVs and receive the sensed images. An Online Message Relaying technique (OMR) is proposed to schedule the relay UAV to collect images. Without any global collaboration between the relay UAV and the survey UAVs, OMR utilizes a markov decision process (MDP) that determines the best schedules for the relay UAV such that the image acquisition rate could be maximized. Evaluation results show that the proposed relaying technique outperforms traditional relaying techniques, such as the traveling salesman problem (TSP) and the random walk, in terms of end-to-end delay and frame delivery ratio.