I don't have PS7 but in Elements 5 I use the elliptical marquee tool to create a selection around the photo then inverse and feather by 250. And use levels to darken the corners.
If you are using CS2 you can add it in the RAW file in the Lens tab. If it's a .jpg then go to filter, distort, lens correction. You can alter the strength and the 'centre point'. My favorite part of PS at the moment