Free images results:
 

Yawning pictures, stock photos and images

 
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Dog Stock Image
Dog
Author: Ichtor
 
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Also, here are some commercial RF-licensed images that match your search criteria:  
Yawning Stock Photos
Author: Plmrue
Cat yawning Royalty Free Stock Image
Author: Zts

Explore more commercial options: More Yawning Stock Illustrations & Vectors | More Yawning Stock Video Footage