Thank you both for your help. I opted to follow Curmudgeon for two reasons-he sounded more like me and I did not want to have to manually position the cursor. My procedure is/was:
Open Paint.net, open image (which in my case was 3456x2304)
Choose Rectangular Select (top left in Tools Window)
in Selection Mode just above that, choose "replace"
Enter Fixed Size 3300x1650 (PIXELS) in my case
Move the selected area around to where you want, then press "Ctrl Shift X" which now makes my "final" image 3300x1650
Select Image > Canvas Size > by Absolute Size and enter 1650 width (half the width of image)
Choose Anchor and select LEFT (left being important)
Now save the resulting image/file under a name you like eg, img_name_left.jpg
Click UNDO
Click Image > Canvas Size and enter 1650 (which applies to this example)
Select Anchor RIGHT (big key point here)
Now save this as img_name_right.jpg (or as you prefer)
Have a soda pop or other.
You should now have two equal halves of an image that does not require an "jiggling" or manual adjustment of your mouse.
For some reasons unknown to me, sometimes (*), it will appear in Windows Explorer that the image sections have not been cropped. When used in the application though, things seems fine.
I do not believe this violates any forum rules and I am NOT promoting, but I am using this in an application where a demo can be seen at:
http://www.flashpageflip.com/demos/free/
If so, I apologize and mods, delete as appropriate.