Lo and behold, Istock manages to screw something up again.
All of a sudden, upload is broken.
And this message from Lobo is hilarious:
"We only provide FTP uploads for Video. We haven't ever provided FTP for the other file types. I have no idea why we haven't ever considered FTP for photo
but I'm sure there is a good enough reason considering we haven't had the option thus far."
Every other microstock site has FTP, but iStock has a "good enough reason" not to use it

(Failing IT department, perhaps.)