## 1 Introduction  
Wireless sensor networks (WSN) are a class of _ad hoc_ networks where resource-constrained sensor nodes are deployed for some kind of monitoring or control function. A typical configuration of sensor nodes comprises one or more sensing units, one processor, memory, a communication component and a power source. Such sensors will then be used to perform measurements of some physical magnitude from the surrounding environment [1]. Those measurements are processed and transformed into electrical signals, which are finally transmitted using the communication component over a wireless channel toward the sink node, supported by a set of protocols and communication standards. WSN that gather visual data from a monitored field operate under the same principles, but visual data sensing, processing and transmission are more challenging due to the huge amount of information to be handled when compared to scalar data. In general, sensor nodes have processing, storage and transmission limitations originating from their resource-constrained nature. Camera-enabled sensors deployed to retrieve image snapshots and video streams will typically demand more resources than traditional scalar sensors, bringing additional challenges to the design and operation of wireless visual sensor networks (WVSN). In recent years, many works have proposed innovative solutions to enhance the performance of those networks, presenting promising contributions [2,3]. For some of them, sensing and transmission of image snapshots are more feasible than sensing and transmission of video streams, defining the scope of wireless image sensor networks (WISN) [4,5]. Many WISN applications will have security requirements. Sensor nodes may be deployed in large and hard-to-access areas, where the wireless channel might be accessed by unauthorized people. In addition to inherent problems when trying to assure confidentiality, the transmission flow may also be subject to integrity attacks. At last, authentication is also required for many applications, in order to assure that retrieved information comes from valid source nodes. The resource-constrained nature of typical WISN applications discourages the use of traditional security mechanisms as those employed on the Internet [6,7]. Strong cryptography, for example, may rapidly deplete the limited energy supply of sensor nodes. As an alternative, some works have proposed innovative approaches to address these issues, employing optimized solutions. Wireless sensor networks have many vulnerabilities that could be exploited by intruders. Thus, we initially describe common vulnerabilities in wireless sensor networks, which may also affect wireless image sensor networks. Confidentiality, integrity and authenticity can be often assured by data encryption, which is used as a basis for many security approaches. We then state the fundamentals of data encryption in wireless sensor networks, indicating promising approaches when dealing with image sensing. More specifically, we survey the main issues related to symmetric and asymmetric encryption in wireless sensor networks and how such paradigms relate to image coding. Moreover, we survey research works covering different aspects related to image security in wireless sensor networks, addressing selective encryption and watermarking. Additionally, secure image monitoring in wireless sensor networks is also reviewed, since it can bring significant results to those networks. Finally, we present promising research areas that can guide future research efforts, highlighting expected challenges and benefits. To the best of our knowledge, such a research review has not been done before. The remainder of this paper is organized as follows. In Section 2, we present security issues in the wireless sensor network context. Image cryptography is surveyed in Section 3. Section 4 covers selective encryption of images. Watermarking in wireless image sensor networks is reviewed in Section 5. Section6 addresses secure image monitoring.