Ok, I only worked on this for 5 minutes, so understand this is a quick example, but I'd start by cutting the render out of its background (if it's not already a layered file) using the pen tool (or lasso if you're more comfortable with it). Keep both layers though and add them to the background image. Adjust them to the size you need (making sure they stay the same size). Then add a layer mask to the bottom layer of the render. Depending on the effect you want you can use the gradient tool on the layer mask (using black as the foreground and white as the background) to add the blend you want. try experimenting with different types of gradients and different transparency settings for the layer to get different effects. Again, this was done with this technique in just minutes and is probably only 1 of several methods to obtain this effect:
Hope that helps a little. I can try to put together some screen shots if you need help trying this.