According to the Operation Manual :
You can save screenshots by inserting a micro-sd card into the card slot & pressing the Power button, then the Zoom (+) button. (you cannot save screenshots on a map card)
This is the same way I save screenshots on my Lowrance Elite 5DSI unit. The unit only has one card slot, so if you have a map card in the slot, you'd have to remove it & replace it with a blank micro-sd card. The map cards are "read only", so they cannot be used to take screenshot images.
... cp