Hmm ... its diffcult to be 100% sure, but its also very difficult (although with some weird data compression algorithms its not impossible) to accidentally vary localised pixellation across a simple photo.
However, aligning a camera, or even just manipulating an image to have perfect verticals as well as perfect horizontals is very skilful ! (all images are captured using a lens - virtually all of which have some edge distortions)

so maybe you guessed right !