Some time ago there was a Windows tool that would let you do this. I never got it to work on my site. I wish I could remember its name, but unfortunately I cannot. I have not heard anything about it in quite a while, anyway. I doubt it would work on the current Zenphoto version anyway.
I think someone else was working on an e-mail upload.
Maybe some judicious searches of this forum will turn up something useful.
Otherwise, I suggest you find a user friendly FTP client for them. I know that my sister can easily use Filezilla, a nice windows based client. With it you can drag and drop. I also used to use windows directly to drag and drop to a network location that was an FTP site. Problem with this was that the connection management sucked so I was always running out of connections and having to wait until they timed out.
If you are on the Mac side, Panic just released Transmit 4.0 there you can create folders that will automatically upload anything you drag onto them via FTP/SFTP/S3 and more, making them look like iDisk's very neat feature indeed.
My ftp client Transmit (for Mac, not free though) also lets you drag and drop as does the free ftp client Cyberduck (also Mac).
if you want to do such a plugin that will be welcome as any contribution is. I have no idea if that even is possible purely webbased. Maybe there is already something for jQuery out there.
Indeed, a user did a email-upload-plugin: http://www.zenphoto.org/2010/04/3rd-party-plugin-zpimpmail/ (I haven't tested it)