With the "mailto:" form? (When I tried to use the one on Lissa, instead of openning Outlook, it openned YAHOO! ...Did I do something wrong?

It'll open whatever your default email client is...like mine will open in hotmail. The basic code should look like this
<a href="mailto:you@site.com"><img src="image.ext" border="0"></a>

Edit: sorry...you mean the formmail thing dont you? Either way it just opens the default email client.