How do i tell her how pretty she is(pic)?

Okay this is Emmas step sister. i sorta went on her laptop ..she left it on.. and i took a picture of her.

Shes been bullied her whole life and dropped out of school. She has a horrible self esteem and i want her to know how freaking pretty she is. i know she will kill me for posting this. but i think shes pretty, how do i tell her, everytime me or her boyfriend do she denys it. random people walk up to her and say shes pretty but she gets sad beacuse they think there making fun of her. and she never takes pictures seriously she thinks she looks horrible what do i do?

here is a pic of her(she was talking) and sorry its not a good pic of her and shes not wearing makeup and she was reaching for sour patch kids and pointing at me but ive always have looked up to her..i think shes gourgous. i just wish she would see it..