image on the left is the premultiplied image, as you can see it
has a black 'halo' around it. This comes from the black background
that it was rendered against. The non premultiplied image is correctly
comped over the photo. When we composite in Photoshop this way
it expects to get a non premultiplied image to work with. I've
been unable to find a way of working with premultiplied images
this way in Photoshop (I'm sure you can, and if you know how,
please let me know).
If you take your images into a
program like AfterEffects you have the option to specify wether
the image is premultiplied or not, and if it is you can specify
what the background colour is - this is important. AfterEffects
will then make the correct adjustments to give you a clean comp.