Images with different thumbnails

I'll just stick this here as I don't really know where else to ask, but how are those images made that have one thumbnail, but the image is something else. The full sized images has gaps between the pixels, so I think it abuses whatever 4chan uses to scale down an image to create thumbnails or something.

I can't explain how it works, but I happen to have an image guide that tells you how to do it. I haven't tried it before myself. Please don't abuse it to make overly distasteful or uninteresting images.

tablecat.webs.com/1349480256022.jpg

http://www.thewebcomiclist.com/o/8538

http://www.thewebcomiclist.com/o/8538