Why do the pictures I have linked to my contacts keep disappearing?

[AC Question]

Several times I have selected a picture saved on my phone to appear next to a contact name and after a few days most of them go away. I have repeated the process multiple times, but they continue to disappear from the contact name. How can I save them permanently?

 

[B. Diddy]

Welcome to Android Central! Which phone? Are the photos that you select stored on an SD card, or on Internal Storage?