Start with any picture. It can be in color or black and white.

Next, select your gradient tool.
Next, select the rainbow gradient.
Create a new layer (control+shift+n)
In this new layer, click the bottom of the picture, and hold the click, and make the line stretch to the top. Your picture should now look like this.
Now, change the blend mode to Overlay (Layer >> Layer Style >> Blending Options). Set Blending mode to "Overlay" (or whatever you think looks best.)
There you have it!