What about, you send the photo to upload(a)wikilovesmonuments.org
(one email address, easy to publish)
Then, after N minutes (30? 60?) without sending us a photo, we reply saying:
You have 4 photos queued for upload. Please visit when you have time
http://www.wikilovesmonuments.org/upload-queue/bc781acb3c8d4ab39461675feabb…
in order to finish the upload.
And we query all the details there.
We expect them to be logged into commons and have javascript enabled.
The low-level transfer could happen through Upload by url (whitelisting
wikilovesmonuments.org).