Thanks enigma1 for your comments.

I think only people with technical knowledge, and only a few set their browser the way you do. There are however more people who display my images in their blogs.

I did not know about the solution called "cold-linking". It does sound much better than looking at the referrer with htaccess and I will look into that.

I don't think I will use the zen_db_prepare_input and zen_db_input functions to filter the form data. My idea is to come up with a form which is useful not only for stores but for any other site as well.

I found this page about filtering :
http://myphpform.com/validating-forms.php
and I'll try to adapt it to my needs.

Thanks again for your help.

Denis