Reducing transmission energy consumption is one of the most critical research issues in the area of Wireless Sensor Networks (WSNs). In this paper, we present our scheme for employing collaborative signal enhancement to achieve energy efficient image transmissions in WSNs. A collaborative signal enhancement approach is compelling due to its capability of saving individual energy consumption by spreading total transmission consumption over multiple sensors. Individual packets describing an embedded wavelet-encoded image exhibit a significantly unequal contribution towards image quality. By leveraging this fact we develop a strategy of appropriately selecting the number of collaborative sensors for each packet transmission in order to achieve the highest possible image quality given a restricted transmission energy consumption budget. Experimental results show that our proposed approach can provide about 2dB higher image quality under the same energy budget compared with the state-of-the-art collaborative transmission scheme in WSN literature.