Why is my startup disk so full?
It turns out that another user account (not the one I was logged into) had many gigabytes of photos stored on the hard drive. DiskImageSweeper, along with any other tools (spotlight search, etc.) could not search through the other user's files, so they were virtually undetectable.

I solved the problem by logging into the other user account and clearing up space. I imagine that you could also just delete the user's account and have the same effect.