I present to you: the ugliest tutorial EVAR
This is a tutorial on how to stitch images into a triptych in Photoshop. I have Photoshop CS2, but this should work in any program that has layers, or you could probably figure out how to do it in a program that doesn't have layers. It will also work for any number of images, but I was asked about how to make a triptych.
Stock images used in the example are my own.
Download for full size, though I don't think you'll need it that big.