Ok, here's the update on the re-install:
I've uploaded the freshly-downloaded ZP files (v 184.108.40.206),
to a separate folder.
This time I kept the default name "/zenphoto", in case renaming it had mucked something up last time.
It appeared to work smoother than the previous time; There were no PHP/MySQL warning at the very top of the page, as there had been at first the previous time.
Also I simply ignored the "warnings" about magic-quotes, etc.
As long as it was "warnings" and not "errors".
Used zp2_ as the database field suffixes, so as not to interfere with the first install.
Didn't delete setup.
Created a user, and added an image to a new gallery.
Same problem, the picture is 403-forbidden, though the listing is there, and the file is definitely on the server.
Went back to setup, changed the permission settings to the less "strict" one (755 as I recall - planning to work my way down the list), hit apply.
Now... everything in the /zenphoto folder comes up as "500 Internal server error".
I can't see any more: the set up page, the admin area, not the gallery.
But interestingly, in my FTP client, the 3 folders are now (seemingly) set to 755! (in this second install -- the original install is still wrong).
Tried manually setting the permissions in my FTP client, but now it's just stuck on "500 Internal Server Error".
Does this sound familiar & resolvable?
Or am I just making life hard for myself with 'Easily' as the host?