On Photoshop CS3, How do you change Opacity on the image

PhotoNewb

I'm trying to make my own DIV layout.
But I want to decrease the opacity on PART of an image, not all of it.
How do I do it?

http://i175.photobucket.com/albums/w136/Lilyxo1/sdfsdf.jpg

Look @ the top left picture of Taio, the background layer overlaps it, and makes that little square thing on the side of his face, which doesn't look good.

Before, I had it, but after I save it [merged down] It became see through, for some strange reason.

DARK-TROJAN · Posted: Fri Jul 04, 2008 11:40 am Post subject:

dont apply opacacity

just cut his face and put it right there it looks better than with opacacity

i hope you find a solution :}