Importing Paper Maps - Rotational Alignment

I have a large number of maps that are physically drawn on paper.  I can naturally scan the maps in to create image files... but it is virtually impossible to get the grid perfectly aligned so that it is exactly straight horizontally and vertically.  Is there some utility that folks use to rotate their scanned in images ever so slightly so that the grid lines end up being perfectly vertical & horizontal?

Aside from being very careful when you are scanning? If you are using Photoshop, it does have some alignment tools, but in general, the easiest solution is to get it as close as possible, and then turn the opacity of the Roll20 grid down to zero. No one will notice if the grid is off by a few pixels if they can't see it.

I usually turn off the Roll20 Grid in situations like these.  It's nice that tokens snap in and such, but not entirely necessary for play. The drawn grid will still do the trick.