How can I overlay a rainbow gradient over a photographic image in Photoshop?

1. Convert your source image to greyscale. (Either by desaturating it or converting the image mode to grey scale and then back to CMYK/RGB.)
2. Add a layer above your image.
3. Fill the new layer with a gradient of the colour and angle that you desire. (The example image is a diagonal rainbow.)
4. Set the gradient layer to MULTIPLY in the layers pallette.

Done.