Basics: Playing with Opacity

Open two images the same size or resample them to the same size.
Select the image that you want to be opaque.
Click on Selections/Select All/
Click on Selections/Modify/Opacity.
Adjust opacity settings to 50% and click ok.
Press Ctrl+C to copy.

Activate the image you want to see through the opaque image.
Press Ctrl+E to paste.
Right mouse click to deselect.

Another interesting opaque effect can be achieved using the clone brush.

With both images open, select one.
Select the clone brush. Settings: size 30, shape round, opacity 10, paper texture none.
Right mouse click on the selected image
You will have to play around with the align/non-align setting to get just what you want.

Activate the other image
Pressing the left mouse button, run the clone brush over your image
Be careful to apply the clone only once to any given area as multiple applications will lessen the opaque effect.
You'll have to play around with the clone brush to get the 'feel' for how it works.

The title banner is both finished images transferred into a third window using the clone brush, opacity set to 10. The text was added with the floating option deselected, moved to one side slightly, and a drop shadow added to finish it off.