Well normally, I have to take my pics at a slight angle in order to reduce glare from my lights and to get the lighting right...kind of a result of my ghetto set up. Plus the curvature of the egdes from the lens distortion. I have ALWAYS been using the Distort - Lens Correction feature in photoshop to adjust the bowing of the edges of the painting. Then I go in and use Transform - Distort to get it square and straight and rotate.
Just yesterday, while I was in Lens Correction, happened to glance down past the Lens Correction sliders...AND THERE WERE THREE TRANSFORM FUNCTIONS!!!
Rotate! Hortizontal AND vertical perspective correction! HOLY CRAP WHERE HAVE YOU BEEN ALL MY LIFE!!!!
Here is the difference:
BEFORE ANY CORRECTION: