That might work if she’s recognizable in the photo and the church that did the baptism is considered (by the Catholic Church) to perform valid baptisms. I think ultimately it’s up to the parish as to what is acceptable proof and if the baptism was valid or not. I was (more than likely) baptized as a baby in the Methodist church (the baptisms of which are normally considered valid), but I could not find *any *paperwork. And boy, did I look for it. No one who might have been there at the time was still alive, so I was unable to provide proof of baptism. I will have a conditional baptism when the time comes for confirmation. I’m looking forward to that as I’ll remember it this time (plus I’ll finally have “written proof” when it’s done)!
BTW, it’s worth looking for some sort of proof of baptism because if you can find it, it should make for a smoother RCIA process. You would be considered a “candidate” from the get go and it might shave some time off of your instruction.