I think I see a lot of barrel distortion in that photo, making it difficult to draw any conclusions about what is level. That's pretty typical for the low-cost zoom lenses in digital cameras, to go from severe barrel distortion at one end of the zoom to severe pincushion distortion at the other. Some manufacturers try to correct this in post-processing of the digital image, looks like your camera does not.
With this much distortion, I think it would be pretty difficult to say much about what is level based on pixel coordinates. Don't pick just two points, follow the edge of the building roof all the way across. I think the change in Y coordinates as you go across is going to show you the roof as bow-shaped, opening downwards. Any two points, even if the camera is level, Y is going to depend on where you are on the distorted curve.
