The first snapshot showed the singer tightly hugging him for a sweet kiss with the caption- "I can't wait to marry you! Best 4 years of my life!"
Simpson, 33, and Johnson, 37, also spent an artistic afternoon looking at the art inside the Tom Everhart Studio in Venice, California, reported E! online.
"I am so honored to be the mother of your babies and loved by you every moment," Simpson said in the caption.
The couple started dating in 2010.