Often it is helpful to be able to extract the video, image, and music data that is being served through the browser. If you use Chromium under linux, this is really easy. This includes almost all of the images you view, as well as video data that comes from Youtube and other videos you watch.
The cache data for Chromium is stored in ~/.cache/chromium/Cache in each user's home directory. Browsing to it in Dolphin will allow you to preview the images, and open the files to see what they are. Unfortunately each of the files is renamed to some type of hash used by Chromium, so you will have to use previews, or guess as to the contents of each file.
In the future, I am considering creating a tool that automatically runs ffmpeg2theora on any image file it finds larger than 50MB (these are typically the only ones I care about), and saves them for later viewing.