This question comes up in the GD&Q fairly often.
Open the "Before" picture in Paint.NET.
Set Units to Pixels
The pixel dimensions of the image are shown at the bottom right, in the status bar.
Click on the menu item - Image
Click on Canvas Size
Be sure that there is no checkmark at - Maintain Aspect Ratio
Double the Width number.
Example - 722 X 2 = 1444
Anchor - Left
Import the "After" image.
Layers > Import From File
This will bring the "After" image in as a separate top layer.
The "After" image will automatically be selected, and the Move Selected Pixels tool will be activated.
Use your mouse to move the "After" image to the right.
To make fine adjustments, use the keyboard Arrow keys.
Once all images are positioned where you want them,
File > Save As...
Save as whatever file type you want.
This tutorial shows how to place two images equal in size next to each other.
If the images are of different pixel dimensions,
you may want to resize and/or crop the larger image before starting.
Edited by Sarkut, 08 January 2011 - 11:57 PM.