Mockups on top of existing images

The most common use-case I have for mockups is to modify an existing UI. The way I would most like to do this is to take a screenshot of the current UI, photoshop some extra space where I want it, and then throw that screenshot into Balsamiq so that I can add balsamiq mockups on top of my image. Please consider this feature, as without it I will turn to Photoshop in place of Balsamiq 90% of the time that I need to do a mockup. It'd be even better if Balsamiq incorporated simple image editing- maybe I could cut out Photoshop all together!