It has nothing to do with insecurity. It has to do with Respect.

Nemo, you and I are in similar positions. My Girlfriend has 2 kids by another guy. I wouldn't have a problem with pictures of him being around, but she wouldn;t disrespect me that way.

If this becomes an issue, you need to sit down and talk to her, explain why you feel it is disrespectful and if she doesn;t understand and can't be somewhat respectful and remove them, then it may be time to go your seperate ways