My girlfriend texts other guys and it bothers me?

my girlfriend will text other guys, who are also my mates! and i dont like what i see. they are flirty and sexual and she says she is just joking around but i dont find it funny. it has happened a few times and i have checked her inbox and found a naked picture of another boy that i know in her inbox. i really love her to pieces and i dont know what to do. i have told her that i dont like her texting these boys. i said "i havent got a problem with you texting other boys. what i have a problem with is the way you text them. the flirtyness and the sexual manor of texts. please can you stop? you would like it if i sent girls texts like you have been sending boys" she told me she would stop. then after i told he this, a couple of days later, thats when i found the naked picture of my 'friend' in her inbox. advice please........