Can we 'twist' a digital picture to be square with the camera?

[maryanne kehoe]

What 'trick' am I missing to take a photo of my watercolor paintings?
Can we 'twist' the digital photo to be square with the camera?

I must be doing something wrong. When I edit my digital photographs of my
watercolor paintings, I need to crop the photo but I can NEVER get the crop
edges to be exactly square with the digital camera photograph of the
painting.

Therefore I always find myself losing a few millimeters of painting so that
the photograph is square. What am I doing wrong?

Here is how I take the picture.
a) I put my unmatted 30 inch by 22.5 inch watercolor on a white towel lying
on the ground
b) I stand over the painting, sometimes on a step stool, and position the
Canon PowerShot A95 5MP camera over the center of the painting
c) I snap the shot using flash & macro focus making sure I am as square as
possible and taking an extra inch or so of border all around to facilitate
cropping

Back inside the office, each and every time, when I crop the 2592 by 1944
pixel digital results using Irvanview 3.98 on Windows XP, I end up with
edges that are not square at first. I have to lose some of my precious
painting just to get square edges when I crop.

What am I doing wrong?
What's the trick to get a photo that is head-on with the painting?
Is there a way to twist the painting with software so I don't have to crop
edges and lose painting data?