I keep trying to tell her how much I love her and how great she is and her only response is always just "nah". I even tried to send her a picture of herself that I saved on my phone (which I found to he a very gorgeous picture) and she got even more depressed saying "I dont like to look at myself".
I dont think shes depressed or anything, but it's starting to really bring me down, seeing someone I cherish so much feel so down about herself

Some people need reinforcement from others. We all like a compliment now and then and some people sort of feed off them (no negative connotation implied.) While it's good to inherently like yourself and feel confident, some people just don't, justified or otherwise. She may need to talk to someone about it, but 90% of the time what therapy does is help you live with a difficult issue and teach you tools to navigate it, and it seems like she's already dealing with how to live with a negative issue. And honestly, lots of people that are brilliant or beautiful or talented are very, very insecure.

I think she would benefit from Jordan Peterson's second rule
Google it mate
It's really hard when you love someone and they don't love themselves

It's lovely that you care for her so much and I'm sorry she doesn't have good self-esteem and self love. It's tricky dating someone like that but please don't fall into a common trap of becoming her only source of self-confidence or therapy. Your job is to love her, not to make her love herself. As mean as that may sound, self-love is exactly that, from the "self". If she doesn't believe that she's a gorgeous person herself then it's going to be extremely difficult for her to believe other people regardless of how truthful they're being. I think it's important for you not to try and force her to see how great she is but rather be subtle and consistent. I'm sure you already do this but just remind her "you're beautiful" or "I love the way you always think about things like that", just some gentle comments added into conversation and slowly hopefully she'll start to see how wonderful she is.
Have you suggested she talk to somebody about it?

Not yet @parisqeen. Maybe I should.

I wouldn't push it if she isn't comfortable with it but maybe suggest it if you think she'll listen.

Self image issues are a rather difficult subject for many people, and I think the best thing you could do is support her and encourage her-which I’m certainly sure you already do. But like other folks said don’t lose yourself. Maybe tell her what makes her so great if she is comfortable with it, if not the advice other folks posted is A+!