When editing a page to add an image, my Insert Media overlay looks normal, but there are no sizes to choose from in Attachment Display Settings. The Alignment menu is right, and the Link To menu is right. There are no options in the Size menu.
I'm running 4.6.1.
I've ruled out active plugins causing problems, but have not ruled out interference from a plugin having been installed in the past.
The server (over which I have no control) has GD enabled.
Different sizes of each uploaded image are being created as usual. The images are there, but do not appear in the Size options menu.
I'm running a custom theme that I built from scratch, so I'm comfortable that I probably haven't caused this one myself. There's no custom code, in functions.php or anywhere else, that changes anything related to images.
Has anyone run into this before? Thanks in advance.