My best friend is a pretty girl with an ugly heart

So I am about to start my sophomore year of college, at the beginning of my freshmen year I met who was to be my best friend throughout the whole year and probably most of my life. She's great. We laugh together and are inseparable and she is there for me no matter what. The only problem that I cannot stand about her is she constantly talks bad about people. It's never really close friends so I used to not let it bother me but it has gotten to the point where it is so annoying and makes me not like her negativity and not want to be around her. She is a really pretty girl but I know she is insecure about her weight (she's not overweight but she constantly thinks she is) so i think that's why she makes herself feel better by talking about people. She will literally be scrolling through instagram right next to me and say "Oh my god this girl is so ugly" probably 3 times in one sitting. She will constantly show me a girls pictures and say "She is too fat to wear that" or "Her teeth are so ugly" or "She is not pretty at all" and I never know what to say. I hate talking about people's appearance because it really is just mean. But i feel like if i ever told her that she would immediately feel stupid and get upset. Is there a way to subtly get her to get the hint that I don't wanna hear it, or that it is mean that she says that? Not saying anything doesn't work, usually i just don't respond but she still does it. Help?