holy shit OP!!!
Ok you now have a few choices...which ever you decide will effect your roomate as well somehow.
-Continue to monitor her (leave the camera alone) and see what days and time she does this. Write everything down (example, came in Thursday at 3:30pm, left at 3:15pm at such and such snack). Do this for a week.
-If you decide to tell your roomate about this just with one camera picture, he might get pissed saying that it might just be that one time and it was her last time (he's going to protect her)...be prepared for a heated argument.
-If you decide to hold off and monitor for a week and develop a pattern of time and days she comes in you could either
A. Decide to stay home around that time and catch her in the act opening the door with the key and confront her.
B. Talk to your roommate during a good time (not when he's stoned or drunk!!!) and tell him what you have learned. Remember it is HIS GIRLFRIEND.
Let him deal with it. You, him and her could all sit down and talk about this and figure out what's going on.
When i said it is going to affect your roommate, well he is going to be troubled that his girlfriend is coming in not only snatching your foods, but by burglarizing your home.
She just committed a crime by entering the apartment without authorization. Finding out who gave her the key is the next step, whether it was the maintenance people giving her a copy, your roomate OR she somehow managed to copy the key.
If it's the maintenance people who gave her the key thinking it was ok to give her the key because she was your roommates girlfriend, then you need to tell your landlord because you gave (nor your roommate) no permission for this to happen.
If it was your roommate and he forgot that he gave her the key, well then you need to get that key back.
if she got a copy some how from your roommate, then you need to get that key from her some how.
OR if you want to be spiteful, report it to the police. They will need just a bit more evidence before they can press charges...so a few more days of monitoring her would be good. That is the last resort.
One final option, depending on how comfortable you are at doing it...
Talk to her when your roommate is away...not in an accusation mode, but in a "I know what's going on, but I am not mad, I am just here if you want to talk about it because I am your friend."
If you are comfortable about this, this might end up well without your roommate learning about it and she *might* just confess to him later on.
This situation will end up hurting your roommate IF it comes from you... Let her get caught by him OR let her confess to him and let them handle it. It's not your relationship, but it is your food. Tell me is a few snacks worth loosing a friend rather then helping them?