Every deck has a gallery: its own set of stored pictures, listed on the deck's Gallery surface and reachable from any card in it. Adding art to a card is choosing from that gallery, or adding to it and then choosing.
Why the deck holds them
The alternative is a picture attached to one card, which sounds simpler until the second card wants it. Then it is either uploaded twice, stored twice and corrected twice, or it is quietly shared and the card that deletes it takes it away from a card its designer was not looking at.
Holding pictures at the deck means one image can sit on as many cards as you like, and removing a card never removes the art you made for it.
One word for one thing
Gallery means a deck's own pictures on this site and nothing else. Browsing what other designers have built is Explore, because that names what you are doing rather than the room you are standing in — and the two words together cover the whole territory without ever overlapping.