The folder was automatically set to 777. I tried reuploading, and that did not work. When I download the images from my FTP client, they show up just fine on my computer, too. So I do not think the files are corrupted. I even tried totally remaking the item_images folder from scratch, adding the items, and chmoding it to 777. The images still do not show up.
Images in all other folders show up just fine.