Basically, download the trial and install it. Then open Help and go to Geometric rectification. Follow the steps and guess what? You can use a single photo of a building as a scaled, rectified background to help you model an existing elevation.
Unlike Photofly / 123D Catch, you don't need a stack of images to make this happen, just one.
You can then use Export button to export a DXF file with a PNG linked in - all at the correct scale. To bring this into Revit, I just drafted some lines in AutoCAD around the image frame in the DXF and exported those 4 lines to a DWG. This was linked / imported into a Revit view. Then, I placed the PNG in the same Revit view and used the 4 lines to scale the image appropriately.
The only real caveat with the Trial version is that it watermarks the image severely - but if you just want to use it as a reference, it is fine.