The Foundation Image Slide stack is an Impact Child stack that allows you to add a Foundation Image as an impact slide and benefit from the interchange loading of different sized images for different devices (just like the normal Foundation Image stack does)
You cannot drop the Image Slide onto a page. You add it to Impact via the add child + button to add a new slide.
The only reason that you are seeing it in your library is that you have enabled “Show Hidden Stacks” on the cog menu of the Stacks library. Doing this will mean you see all hidden child stacks which may well get confusing. It is better to turn off “show hidden stacks” for normal use. They are hidden for this very reason to avoid confusion.