How do one save a file as a gradient?

Hello, just joined and taking part of all the lovely tutorials. There is only one problem: A lot of these also requires the use of special gradients which comes with the tutorials, and I have NO idea on how to SAVE an image into Photoshop as a gradient. I have found out how to MAKE gradients by checking the memories, but how to take a finished image in JPG-format and make it into a gradient, I am totally at a loss.

Help is much appreciated. =) And a newbie-slap probably well deserved. >.>

EDIT: I'm using Photoshop CS. *slaps self*