Most Helpful Guy
It's because they once had a good relationship with them and want something as a memento to remember those times. I'd like to think a lot of people would want to move onto their new relationship and discard any materialistic possessions of their previous lover, but some people can't let go of those memories they once had with their ex because it touched their heart so deeply. People get attached and it may take a while for them to move on depending on how healthy their previous relationship was. That's what I would imagine the majority of people who keep their ex's pictures do, however everyone is different!
Most Helpful Girl
It means they like their ex’s picture