The Android WebKit rendering pipeline is summarized in this document. The final section provides a patch to the android source code that will log all the rendering activity. A sample browsing session is traced with this code, with all image-rendering activities logged and offline processed and rendered with a standalone installation of Skia. Each time WebKit updates the screen, an frame is produced from the log, and concatenating these frames with ffmpeg makes this video. In this way, I'm able to log all images that are actually drawn on the screen during a browsing session.
I wrote this code to study opportunities for bandwidth saving -- an image in a webpage won't have to be actually downloaded if it is not shown at all, and only a low-resolution version of the image, given multiple resolutions are available, is needed if only a zoomed thumbnail of the image is shown.