We propose a fast and efficient lossless codec of continuous-tone images, SPEDIC (Simple Predictor and Edge Detector based Image Codec), which is uniquely suitable for coding screen updates generated by multimedia applications in thin client computing systems. A codec for thin client computing should take account of the tradeoff between compression ratio and coding complexity, because screen update images in thin client computing should be sent from server to client in a timely fashion. We argue that by combining similar but simpler building blocks of the state-of-the-art methods, a little inferior compression ratio of JPEG-LS (the standard for lossless image compression today) can be attained with much lower coding complexity.