Some of you might think this is stupid but whatever. Okay so my friend has a boyfriend and a girl posted a picture of him saying I was her crush cause she doesn't like my friend but likes her boyfriend. So I commented saying that the picture was rude because it was just up to hurt my friend. Then on the comments she started threatening me and saying she was going to me up and hit me and all that. Then she messaged my friend and said she was going to come after me. This girl is known to physically fight so I have no doubt she will come after me. Is this something I should bring up to the police? I screenshotted everything and I am actually frightened but don't know what to do..

'my friend has a boyfriend and a girl posted a picture of him saying I was her crush cause she doesn't like my friend but likes her boyfriend' is indecipherable.
It doesn't matter.
Tell her on the comments that you did a screenshot and will report her if you so much as stub your toe.
There's zero chance that the police will do anything. Very rarely are threats carried out, and threats are basically free speech - not a crime.

I think you leave her alone like you should have done when she posted her crap. Challenging a known bully/crap talker is a bad idea and has consequences. Actually the time to be afraid would be before you challenged this girl, as it's a bit late to be afraid after. I doubt the police get involved in the social media conflict, unless something unlawful happens. Learn to walk away from high school drama.

How old are you both? If this is serious enough to consider calling a cop, TELL your parents FIRST.

How old are you both?
I read back and learned the OP is 16 or maybe 17 now.

If this is someone you go to school with, then report it to a teacher. This falls into the area of cyber-bullying and schools are taking a dim view of it. Unfortunately the police are swamped and it is unlikely they will be able to do anything unless she actually attacks you. It's a sad fact that the police are more about catching criminals than preventing crime.

Sad? I don't WANT crime prevention to be a cop job. That's a job for everybody but cops.