To answer my own question--I needed to create a separate ISO repository first and choose that SR. Doing that, I can upload an ISO.

The page's text is a bit misleading....

Going to the he Import->Disk page, after choosing an SR, the page shows a drop target labeled "Drop ISO, VMDK or VHD files here to import disks." However, clicking on it brings up a file browser that only accepts *.vhd and *.vmdk. The page source shows <input type="file" accept=".vhd,.vmdk" ...

This is a newly-installed 2-node cluster, XOA version 5.80.2.

Is this a bug? Or am I missing something?