The recent advent of reverse-image-lookup tools on the Web, particularly at Google, makes this assignment no longer completely relevant. The idea of the scavenger hunt was to use the details in the image as clues for generating your own search terms, and then apply those search terms manually to a search engine such as Google Images.
Reverse-image-lookup tools make this approach (unfortunately) unnecessary. I've tried changing the file format between .GIF and .JPG, selecting one frame from a .GIF animation, and even applying a subtle distortion field to the pixels to make the images not exactly what appears on the Internet. These actions were to no avail. The reverse-search tools are simply too good. Out of the 15 modified images, only two came up from the search with the wrong suggestions.
So, it is with great regret that I am forced to cancel this assignment. It was a good one, and I'll miss it. I think the analytical skills it develops are worthwhile, and we are in some sense poorer for not being able to perform the assignment in the spirit in which it was intended.