How come when i get a picture from a text, it downloads it but then goes away?

[AC Question]

Ok. So when someone would text me a pictures it gives me a option to download it so i download it, but then the little expiration date shows that it expires the same time and the same date it was sent. So for example when someone sends a picture to me at 12:00 A.M July 5th, the expiration date says 12:00 A.M July 5th. does anyone have an answer??