Is hanging pictures a trade (as in a carpenter, plumber or handyman)? Well yes, but only in part. You might say that professionally hanging pictures is 40% a trade and 60% an art.
I concede that there is nothing particularly complicated about hanging a picture on a wall; hook gets nailed in and you hang the picture, right? But beyond this basic action there are many variables. Is the picture hung at the right height (too high or too low), what is the aspect of the image