Transform a trapezium into a rectangle
Solution 1:
In GIMP, it is called the perspective tool. I think it is the same in Photoshop.
There is an icon for it in the menu on the left. It looks like a trapezoid with two arrows at the bottom corners.
Solution 2:
ImageMagick gives you the -distort Perspective option. Like explained in Perspective Distortion:
convert image.jpg -distort Perspective '7,4 4,3 4,1 4,1 85,1 100,1 85,2 100,3' result.jpg
Solution 3:
paint.net will do this - you're looking for the 'layers', 'rotate/zoom' option. just move the ball to get it aligned