1. A computer-implemented method for sending image data to a client device, comprising: receiving, by a computing system, a first request to send image data to a computing device; sending, by the computing system and for receipt by the computing device in response to having received the first request, first image data from which the computing device is able to generate an initial display of the image, wherein the initial display of the image is a reduced-resolution version of the image that is reduced in resolution in comparison to a full resolution of the image; receiving, by the computing system, a second request to send image data to the computing device; and sending, by the computing system and for receipt by the computing device in response to having received the second request, second image data from which the computing device is able to generate a subsequent display of the image, wherein the subsequent display of the image is an intermediate-resolution version of the image that has a resolution that is higher than a resolution of the reduced-resolution version of the image and that is lower than the full resolution of the image, wherein a portion of the second image data was previously generated by subtracting a value for a pixel in the initial display of the image from a value for a pixel in the intermediate-resolution version of the image, and wherein the second image data is configured so that the computing device is able to generate the subsequent display of the image by combining the second image data with the first image data.