Thanks to Dac for the idea and code fragments.
For the first trick, move your cursor over the two photos. Several times each.
This is done by building an array of images, and moving through the array whenever the mouse passes over the image. You have to create the images all exactly the same size. It just doesn't work if any of the images are slightly different in size.
By the way, if you're wondering what that strange three pronged phallic thing is in the set of photos on the right, they're heavily eroded rock formations called fairy towers, found in Cappodocia, Turkey. See my Turkey pages for a description.
This trick switches between two images when the mouse moves over the cursor. This was the original code fragment that I saw on Dac's site and pinched. I got this one working, and then expanded it to the previous array of images. Again, the two images have to be exactly identical in size.
I abandoned this line of effort in May 2001, when I wrote a CGI program to do a much better job. See the Turkey photo gallery for how I did this.