I want to steal a picture of myself from my girlfriend's phone
Can't you take a selfie? Why do you need her phone?
She won't let me look at her phone
Your partner's phone is their own business and has nothing to do with you. You should not be snooping in the phone because it is not yours to look through. You are violating your partner's privacy.
Too bad.
If she find out you stole and unlocked it she might not trust you. If you suspect she is cheating you just need to tell her. If she is she might confess, but spying is wrong.