Transparency into background composite image.

I am trying to render an image (glass) in cycles and have it composited onto a background image so that the glass transparency goes through to the background image i.e keep transparency in the layer before it is applied to the photo image.

Is there a way to have the grey background replaced by the image so the glass renders and interacts more naturally.

I am mainly photoshop based so would normally do this via a screen mode layer in CS6. However I would like to be able to create more interaction between the b/g image and the glass. Is this possible? I need the image to be exactly mapped to a background as it will need post work with the original in CS6.