Games developed with RPG Maker VX Ace are unfortunately not compatible with Steams inbuilt Screenshot feature. Fortunately, there is plenty of free alternative programs that can help you take those snaps, and show off rare loot/creature locations.

ShareX is my most recommended as it is possibly to launch/download through Steam.
http://steamproxy.com/steamstore/app/400040/ShareX/

Greenshot is also a fantastic, and free screenshot program. It is a quick install with 0 ads, as well as configurable key bindings.
http://getgreenshot.org

Screenshots can then be uploaded within the "Artwork" section of the ATE:TH community hub, or on various image upload sites such as imgur.com and shared freely.

Hope this helps some budding space photographer out!