@biscoit,
Some people are sentimental, whether or not they are interested in the person as a mate. I've kept plenty of photos of old friends and lovers and no jealous lad (or by now, old man) gets to worry that I have them. Having someone give her flowers, I presume for the first time, is a nice thing, in memory. It's part of her life. Life marches on and sometimes seems to want to be recorded.
Have you two vowed to be exclusive? You sound quite like you want to own her, which can put some girls or women off. Or boys or men.
I'm not trying to be snarky. It's just that many people have boundaries about others having a say in what they get to keep.
You do sound quite untrusting. Your guess may well be wrong, or, it could be correct. Give her a break and don't pursue this line of questioning with her at this point. I can't tell if you two are sixteen or thirty, guessing somewhere inbetween. Talking is a good idea, but always being suspicious is a problem.
Photos can be keepers. I'm glad I have a photo of a guy and me sitting on a bench near the ocean some months before he went off to the Vietnam war. We weren't a couple shortly after the photo for a lot of practical reasons, and he didn't die in the war, but I'm pleased to have that old photo of
how we were back then.. peaceful, smiling. Photos are a dime a dozen now with all the smart phones in the world, but photos can still often add a kind of continuity to life. Trashing them can be dumb.