There are a number of ways you could do this. Probably the easiest is to include a check box next to each so the user can select one. Just give each check box in the group the same name and different export value, and the user will be able to select at most one from the group, or none.
An alternative is to use buttons to contain the images, and when one is selected, it's border color or background color could change to indicate which is selected. This will require a bit of JavaScript, bit it's not too complicated. If this sounds like something you'd like to pursue and need some help, post again.