Hmmmm, You can just place an image on top of another image and lower the opacity.
Layer one is the background image and whatever image you want to be transparent will be the second layer(change this layers opacity to your desire).
A better option would be using layer mask.
step1..
Open two or more photos
step 2...
With the move tool selected in the tool palette...Drag one image on top of the other one.
step3..
Click on the add layer mask button ...Its at the bottom of the layers palette.Its the second button.
step4...
Press G to select the gradient tool and make sure the foreground and background is black and white...you press D to do this.
step5...
drag the gradient diagonally, vertically or any other way across the image.You should just play around with it to get the results you want.
