Answer: There is no upload option within the portal and that is by design.
To access the container requires the appro keys and access. The portal is not authenticated to the azure infrastructure that way, so write operations are not allowed.
The portal is misleading, because download IS available, but that is the case because this container has public read access.... so the azure portal makes the files available.
The simple answer to the whole conundrum is to publish the container as a file share, and use plain jane file copy operations to copy files up to the share.
I tried to use the Azure Storage Explorer, but only got errors and no path forward. I installed Azure Management Studio (the free demo) and it worked great.
So problem solved, after 30 min of poking it.
(It would have been OK if the above were somehow communicated better by the portal instead of requiring so much thinking. Though I understand it now, it is not obvious why the azure console would have the ability to list and download files, with no ability to upload.)