Graphics/ Photoshop

I have been wondering for quite a while how, in Photoshop, to fade between two images. After all, the gradient tool has no "Image to Transparernt" gradient.
Well, I was making some graphics for a website yesterday and I had to select part of my dog's fur and tile that part of the image as a page background. However, there were no "tileable" parts, so I instinctively feathered the selection so her furs wouldn't clash with each other. When I did this, I noticed a transparent ...