In addition to what elizabyte said, you can also get somewhat better results by tweaking the alpha. If your render has a black background and your image is light, for example, sometimes you'll get dark fuzz at the edges of your render selection when it's pasted because of antialiasing with the black background.

Usually I copy the selection to a new document with a transparent background, paste it and do the following:

-Select All
-Save Selection
-click on the appropriate channel (whatever you named it or the default, Alpha 1) so you see the black and white
-apply a slight gaussian blur to the alpha
-go back to your regular view on the layer
-Load Selection
-Select Inverse
hit your backspace/Delete key - each time you do, a little more of the fringe will be deleted until you get the result you want.

There's probably some faster way of doing this, but I've been using Photoshop since version 2 and am stuck in my ways :)
EDIT - after all that, Select All, copy and then paste to your composite image... /EDIT
.m

Message edited on: 08/06/2005 00:13