The standard setup
Remove one queen from an ordinary 52-card deck. The resulting 51 cards leave an odd number of queens. After valid pairs are removed, one queen remains unpaired. The other ranks can still pair completely.
Our smaller picture setup
The printable has six complete picture pairs plus one Sun with no partner. That is 13 cards: twelve paired cards and one odd card. Do not print a second Sun and mix it into the same deck unless you deliberately want a different activity.
Count before shuffling
Lay out the pictures face up, check that each regular picture occurs twice, and count one Sun. This catches a lost or doubled printout before a round begins. Keep a spare sheet separate from the deck in use.
A distinct matching exercise
If you remove Sun, the cards can be used for a pairs-only activity. Name that change: without the odd card, the ending is no longer the Old Maid ending taught here. Face-up practice keeps Sun visible as the final unmatched card.