I have the following notice on my index page ( http://www.orchidcourt.com ):

all contents, pics, etc, © Tennis Maynard

I have had a number of folks ask for permission to use pics for various
projects. I hesitate to grant permission for other ebayers as i feel a
need to verify their honesty, sales integrity, etc, lest in some way I,
through my pics, become linked to something I don't approve of.

That said, I have to feel that linking to a site is like showing someone
a copy of a pic from a book. I might be selling paphs at a local show
and am asked about a particular species, and produce my copy of a paph
book, turned to the appropriate page.

On the other hand, were I to do such a thing (and I haven't yet), I
think I would feel bound to include some notation to the effect that:
pictures of this plant may be seen on so-and-so's website at: (link)

And even then I would feel a little uncertain about it without permission.

As far as unauthorized use of my pics, I would be more bothered by
direct links than by a link such as the one I describe above, including
a notation that I am not connected in any way with the linking site.

I have not yet refused permission to anyonbe except ebayers who has
asked, and have in fact made a point of thanking them for asking
permission.