One of the more innovative attempts I saw was a site that stretched it's images with editing software and replaced every 2nd line with a new one. Then, when they created a page, they set the HTML code to display the image at 1/2 the actual size, so Internet Explorers algorithm to resize images would remove the lines they added, and the image would appear perfectly normal. But if you attempted to save it to your hard drive, and view it on it's own, you would see what appeared as the original image with their logo overlayed.
Not sure if I explained it exactly right but I thought it was pretty neat.