Photoshop :: Fading Image Edges To Black

Each page is just an image file, and the page background is black. So for the image to go into the page seamlessly, the edges have to fade to black. So far I've only been able to do it with specific kinds of pictures that are already black, or have very dark edges, and I just use the burn tool to fade it to pure black so it blends in nicely. If you go to the page, you'll see what I mean. However I'd like to be able to do a similar thing with brighter images, or images where the edges aren't already black/dark. How can I fade bright images to black in a convincing way? When I try using the burn tool, it doesn't bring it all the way to black, and if I use the gradient tool (i.e. from transparent to black or something), it's hard to get it uniform all the way around the image.