Multimedia system is a typical application of time-critical computing. In networked multimedia system such as video conferencing, real-time image communication is the key for system success. To address this problem, we employ time- critical computing technique. In our method, images are decomposed into intensity-information and color information. The intensity information of an image is preserved, and the color information may be spatially sub-sampled, resulting in different colore resolutions of images. When an image is transmitted over network, the intensity information of the image is transferred, then a low resolution color information is transferred. Time permitting, color information of higher resolution for the image can be incorporated. The result is that we can discard a large portion of the color image and let the visual system map the color information into the intensity image. With this method, the image data to be transferred over the network is greatly reduced, and the time-constraints of real-time image communications are satisfied. This technique is successfully used in our cooperative CAD systems.