Right. Start by opening an image, I chose this:

Black background seems to be working best. Select the Circular Marque Tool and select the top of the image where you want the glassy effect, like this:

Right. Now select the Gradient Tool using these settings:


Make a new layer. If you don't know how to do this, it's
CTRL+SHIFT+N as far as I remember.
Now drag, from the top of the image (in the middle) onto where the marque ends.
You image is now done - this was my result:

Looks good, doesnt it?