Yea, sorry for not being clear on that. I uploaded them to the post itself via the "Insert Photo" option when typing up the post (inserting them inline into the post), not adding them directly in the media library
I assumed that resulted in the images being attached to their respective posts, but when I just went back to the media library, all images are showing unattached (which also explains why posts didn't have a featured image). Is there another way of attaching images to a post when I'm writing the post that I'm missing?
Incidentally, I went through and attached each image to its respective post, and now my last post shows a featured image along with a handful of its other images below it, but still no "Photos" sidebar.