Visual information is the most intuitive information perceived by human. Image sensors are able to provide the visual information for recognition, monitoring and surveillance. Networks of visual sensors are the solution of choice for a number of societal, research, and educational applications. Architectural challenges are posed for designers of image sensor nodes such as computational power, energy consumption, energy sources, communication channels, and sensing capabilities. We discuss the characteristics and requirements for an image sensor node. An image sensor node is designed and implemented for wireless sensor network. It includes an embedded processor, memory, a CMOS image sensor, image acquisition unit, RF module and power unit etc. Power-efficient hardware management strategies are proposed for the image sensor node. Image compression problem of image sensor nodes is discussed in detail.