Additional Product IMages

Some of my products show only the one main image, and other products show an additional image.
(in some scenarios I want additional images ,  however sometimes not)

When I try to untoggle the additional image .... I notice there is no option that says "none"
so how do I untoggle and addtional image, so it's not shown on the display for the customer. ?

The same way you select multiple items in any listbox, you also do the same to unselect items:
Ctrl+Click