to reduce the size of html-mails it would be super-nice if only links to pictures etc. could be included; so I mean you only send the html code which links to a picture on a web server.
Would that work? Would the one who receives the mail actually see the picture?
And, how would such a html-link-signature be done in mail.app?

You're exactly right. I'm a multimedia puke specifically for an energy education department at a local college. We send out industry related newsletters and program offerings via e-mail to related individuals & companies that request to get this info via third party energy co-op sort of group.

The last one we sent was to 80,000 addresses; they're all HTML with a text alternative for those that prefer it, and no files are actually sent to the recipient. All images, style sheets etc, are published to an external web server that feeds them to what is essentially nothing more than an E-mail of html code. If you get any of the Apple HTML e-mails change the view of a message to raw source and you'll see they do the same thing.

For those that have taken it upon themselves to decree that e-mail should be text only - if you've reached a point in your life where non-text only e-mail is what drives you crazy - switch to decaf and lose one of your monthly online porn memberships - it'll more than pay for BROADBAND!

Try Command/ i in Safari! ; )