You can use a CGI to display the picture.
(in the page, you put a normal "IMG SRC" tag
but instead of pointing to a picture, it points
to a CGI).

That doesn't prevent people from realy saving
the picture if they want (whatever you do,
you can't prevent someone from doing a print
screen...) but that make it harder. Also,
that prevent people from linking the picture
directly from your site in their pages.
Why ? Because the CGI should check that the
picture is loaded from inside one of your
own pages...

Cheers,
Yann Stettler