I am one of those people who have been surprised that Paint.net didn't have a way to skew, or distort an image. I was lead to the plug-in Distort This!. But I found it difficult to understand how to use it. Now that I have a solution, I thought I would let others know how I use it.
I am taking photographs of some of my artwork, all of which are painted on rectangular canvases, and hanging on walls. I do my best to position the camera at 90 degrees to the canvas and over the centre. Then I get well back and use the longest focal length possible to try to eliminate barrel distortion.
But there is sometimes perspective distortion no matter how carefully I proceed.
It took me a long time to learn how to use Distort This! to correct the problem of distortion. What I eventually did is to use the plug-in on the entire photograph, unwanted edges and all. clicking on the "reset nubs and work area" puts the nubs in the 4 corners of the photograph. I found the handles (nubs) can only be used to pull the 4 corners inward as needed, and can't move outwards past the edges of the photograph. In this way, the sides of the art in the photograph can be made perfectly vertical, and the top and bottom of the art can be made perfectly horizontal as well.
How can one determine when the art is squared up? I've not found a way to put a grid on the screen, but an easy solution is to use the top, bottom, or either side of the Distort This! dialogue box as a movable ruler. Just slide the dialogue box very close to the canvas edge on the photograph and move the nub to create a perfectly vertical or horizontal edge.
I hope this is of some help.