I rarely advertise for paid programs. In this case, the program is still useful in Trial mode, so I’ll make an exception.
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.