My girlfriend added my ex on facebook without telling me and thinks its funny?

No you aren't overreacting. What you aren't seeing, is that if your gf thinks this is "funny", she isn't very respectful of your wishes.... that may lead to a bigger problem, about your gf. I would have a talk with her, and let her know this bothers you. If she still doesn't take your wishes seriously, you have basically only two choices: Stay with her and accept that she doesn't respect your wishes, OR break it off with her to let her know you will be taken seriously by a significant other. I know that sounds serious, but in order to test your relationship with women and see if they are WORTHY of you, this is what you can do. And a woman SHOULD be worthy of you... just a thought. Good luck.

she had no business of posting your ex's pic. I doubt she had your ex's permission to post it, but that's another issue between her and your ex. I see nothing funny about this. to me, this would be a deal breaker.

Who cares

shes being very immature