The subject of how to tell who owns a photo in a copyright dispute came up in Foresnics class the other night. We learned about metadata that's behind the photograph. It's known as EXIF (EXchangeable Image Format). The example showed to us in class had the date, time, type of camera, f-stop, focal length, exposure time, flash (or not), among other data.
If your graphics program doesn't dig this stuff up you could probably find a program to dig it out of the photo if it's there (it isn't always; depend on the camera). From there it's simple trig to figure out dimensions.
I didn't about this. Maybe you guys did.