## 1 Introduction  
In recent years, Wireless Sensor Networks (WSNs) have raised a lot of attention of both industry and academic communities. Typically, WSNs are composed of self-organizing electronic devices equipped with a short-range wireless transceiver, limited energy supply (usually batteries), a sensing unity and processing and memory resources [1,2], addressing applications as surveillance, tracking, disaster monitoring, home automation, industrial control, battlefield surveillance, among others. The use of low-cost, low-power and low-resolution cameras to retrieve visual information of an area of interest can strongly enhance the monitoring capability of such networks, allowing the development of Visual Sensor Networks (VSNs) [3,4]. In general, VSNs are composed of one or more camera-enabled source nodes for a series of innovative multimedia sensing functions, where visual information retrieved from the monitored field in the form of video streaming, conventional snapshots, infrared or thermal images can significantly enhance a large set of monitoring applications [5,6]. Due to the stringent requirements of video streaming over sensor nodes, transmission of still images will be frequently a more feasible option, defining Wireless Image Sensor Networks (WISNs) [7]. Typically, much more energy is expected to be consumed in the transmission of visual data packets over wireless links than in storing and processing operations [8,9]. Images captured by visual sensors are packetized and transmitted to the sink of the network, where data packets are relayed by intermediate nodes in a hop-by-hop many-to-one manner. As packet relaying consumes energy and transmission paths may be disabled due to energy depletion of intermediate nodes, the total amount of information transmitted over the network should be minimized, saving energy with reduction of packet relaying and potentially prolonging the network lifetime. Energy-efficient packet relaying is highly desired in resource-constrained visual sensor networks. A reasonable and feasible approach to achieve this goal is to limit the number of packets that may cross the network. Besides optimizations strategies that early discard packets in source nodes [10,11], packets may be discarded in intermediate nodes in a controlled way according to some prioritization strategy. In [9], each intermediate node decides if it has to relay image packets to the next hop according to its residual energy level and to the packets' priorities. Transferring the dropping decision to intermediate nodes is a flexible solution that does not require constant monitoring of the network condition by source nodes, reducing the overall complexity. The work in [9] assigns a priority level to each data packet according to the relevance of Discrete Wavelet Transform (DWT) subbands. Intermediate nodes check their residual energy to decide if an incoming packet must be relayed to the next hop or silently dropped, based on an optimization policy that associates packets' priorities to energy thresholds. Doing so, the quality of images transmitted from any source node will be degraded when the residual energy of intermediate nodes runs bellow a predefined threshold. However, energy saving is achieved, potentially prolonging the network lifetime. Such relaying approach is especially beneficial for braided-paths, where a single intermediate node will relay packets from more than one source [12]. As hub nodes (which belong to more than one path) will typically receive more combined upstream traffic than other intermediate nodes, these nodes are more critical for the network operation, demanding an energy-efficient packet relaying approach as presented in [9]. We propose in this paper an energy-efficient packet relaying mechanism, where packets are also relayed to the next hop according to the residual energy level of intermediate nodes and predefined energy thresholds.