Dynamic reconfiguration allows to selectively substitute blocks of logic at run-time in order to improve the area efficiency of a FPGA design. This paper presents the design of a JPEG Encoder which exploits this feature. We propose a mixed HW/SW architecture, where most compute-intensive components of the application are mapped to application-specific HW cores. These cores dynamically alternate on the FPGA. Our purpose is to describe a real-world application of reconfigurable computing, illustrating how this approach allows for saving area with negligible performance overhead. We built a fully-working prototype, which demonstrates that the reconfigurable JPEG encoder achieves 29.6% area saving, 1.5% performance loss, and negligible power overhead with respect to a solution which uses statically mapped HW cores.