There is a photo of a handsome colleague in my girlfriend's mobile phone. what do you think?

Probably because she thinks that colleague is more handsome.

Or they have a good relationship.

Then she thought you wouldn't argue with her about it.

So she didn't care.

Appropriate small reminder.

If she doesn't upset you,

Then it's not a bad thing to spoil her a little and give her more trust and indulgence.