How do you Put a render in front of something like a stock?

Well in PS wouldn't you just put the render layer on top of the stock layer?

I think that is what you are talking about.

 

Easy! Paste the stock in, crop it how you like and then take the magic wond over the render, select>invert, press ctrl-x, then go to your original image ( the one with the stock already on it) And press ctrl-v. Then move it around with the move tool and your done.

 

Or in your renders window, edit>copy and then go to your sig and hit edit>paste the go to your layer dialog/window, hit the new layer button (the on to the left farthest out.)
your render will be on a new layer, but make sure you resize it first

 

If you think you are up to it, with a little bit of intuition you can create your own C4D, by copying and cropping the stock a couple of times behind the actual stock.

 

Do the same thing as above and move the layer below the render layer by clicking and dragging it in the layer dialog