I agree with what they said, and will add that the higher resolution photo the better...(hard to place the alignment points when image is too pixelated). But, I think it's good enough to roughly model from then adjust later according to your known dimensions. See if this file works better for you. I aligned to the corner of the desk in the foreground. Not much vanishing point on the red axis, but like I said, should be good enough to help guide in roughing things out.

moje_ambulance - Edited GD3.skp