Whereas the photographs represent Piper's physical transition, the screwdriver represents her mental transition. In the free world, you wouldn't think twice if you accidentally stole someone's screwdriver, right? You'd just give it back.
Prison doesn't work that way, though. When Piper accidentally takes a screwdriver from the work site, she's terrified that she's going to be caught in possessing a deadly weapon. She even makes thinly veiled threats to her fellow inmates who know she has the screwdriver, like when she says, "I don't know what I'm going to do, but you don't know a thing about this, understand?" (6.52). Yup—the stakes are high when it comes to this handy little tool.
Her fellow inmates don't take her theft as seriously as she does, mainly because they're not the ones holding the screwdriver. But Piper's prison mentality takes over. She does take this seriously, and she chucks the deadly weapon into a dumpster, where thankfully, no one finds it.