The dog's mouth is open like he is yawning.
a dog is yawning because he is tired from a walk
A dog yawning does not imply he is tired from a walk.
The dog's mouth could be open in the motion of barking, not only yawning.  There is no statement indicating that the dog is returning from a walk only that it looks like he is yawning, but this does not imply he is tired only that his mouth resembles a yawn.
Dog is not necessarily yawning because he is tired from a walk.