It entirely depends on the situation of the break-up.
For my case, when I see my ex looking pretty I get a sense of pride, coupled with a reminisant mood. I am not wanting to go back there, but I am missing what we had when we had it (but this is based on not having that closeness in the present).. specfically I don't miss her, I just generally missing being close to someone special and feeling things are right (which didn't last too long since we spent most of our time fighting because neither of us could see the other for who we both knew ourselves to be).
If I had found someone who I felt deeply for, who returned the same, I would just be happy for her and smile to think that was someone who once choose me.
She's the ex, her looks were pretty when she and I met, and now her looks are pretty because she's looking for some other company. There's plenty other pretty ladies out there I could be talking to instead of wasting my time looking at pictures of past GFs all dolled up, so I don't pay any attention to what the exes are doing now.
What Girls Said 1
I don't see my ex in person since we live in different countries. So I've not seen him since we broke up 1.5 years ago. However, recently he saw new pictures of me and he told me I was "hot" and then he was really nice for a while. He didn't attempt to get me back though. It also didn't last for very long. Now in hindsight I find it a bit annoying that he's nice to me when he sees pics of me where he thinks I look "hot". It's almost offensive, as if that was everything that he ever liked about me.