It is not designed to read screenshots from the PS4 - in fact, only a very limited number of games actually can. You need to take Dovetail Live screenshots (I believe by holding Square and pressing down on the d-pad) which of course requires you to be logged in to Dovetail Live. These screenshots can be viewed on the Dovetail website and I believe are actually compressed less than the PS4's screenshot function, which is surprising.
