XnView Software

It is often difficult to assess the difference between two images side-by-side e.g. to compare focus. If there was a button (when momentarily held) which overlayed image 2 onto image 1, then you could toggle the difference on and off.
Normal state: Image 1 / Image 2
Feature engaged: Image 2 / Image 2
Feature released: Image 1 / Image 2

You can do like this:

Select the two images.

Go to Tools \ Compare. Now in the Compare tab you should check in the toolbar the „Use tabs” checkbox.

Then you will have just one image at a time on screen. By clicking on the tabs which appear at the left side of the screen you can switch between the two images in order to see what is different.