Clonezilla restore from Samba - no 'restoredisk' option
I think I've figured it out - for local_dev, anyway; tomorrow I'll find out whether this applies to Samba as well. I suspect it does. (Edit: YES!)
When I backed up the Windows machines to Samba, it created a folder for each image. When I tried to restore one of those images, I specified that subfolder to be mounted as /home/partimag; apparently this was my mistake. When I copied the files onto a USB stick, I copied them to the root folder - and, as I mentioned above, it still didn't see anything to restore.
Out of some sort of inspiration/desperation, I created a subfolder on the USB stick, moved the files into it, and tried again - and lo and behold, I have now restored that image successfully! Tomorrow I'll try telling CZ to mount the root of the Samba share as /home/partimag, and I suspect I'll be given a choice of images to restore. (I'll update this answer according to my findings.)
Bottom line: CloneZilla has a blind spot in the root folder of whatever you specify as /home/partimag. You've got to tell it to look (at least? needs testing) one level above the folder containing your image files. This may be spelled out in the documentation somewhere - but I sure didn't see it, and I wish I had.