How can I print screen in game?

Hello,
I´m trying to make some auctions, only the normal windows printscreen won't work in game.
Tried it with dropbox download, get black screens while printscreen in game.
I have no idea how I can make it work so I really need some help here.

EDIT : I am using Windows 10

problem solved, need to use windowed mode

also there is a specific folder where they pop up

you can use fraps too