Basically, after some kind of threshold is passed (perhaps how many images I've opened), the Media can no longer display any image, displaying only the Pictures icon instead.
The picture is still there, and other applications such as Photo Editor can read them just fine, but Pictures cannot display it. This does not happen only to specific images, but to any image, even ones that previously successfully displayed
Additionally, I accessed these images from a file browser, not from the Pictures application.
The only remedy is to reset, either soft-reset or hard-reset.