Then I decided to Show Hidden Files. Immediately, a folder called .Trashes made itself known. Inside that folder was a folder called 501. That folder contained all the images that I had deleted from my memory card via the Finder the last time I imported photos. I was able to delete the photos by dragging them to the Trash and then emptying it.
Moral of the story? Always delete your photos from your camera, not through the Finder.

