Answer:
I had the same question. I think that the answer is we can't with Google's Webmaster tools. I did try adding a HTML meta tag on my Piczo site using the HTML tool. See http://searchenginewatch.com/showPage.HTML?page=2167931 for tips. I just did it today so I don't know if it works yet.
*Piczo has an option to add your site to Google. When in edit mode show your page list. You will see a link that says "Search engine keywords". There you will fill out the form. Once submitted people will be able to find your site via those key words. On that form it also says "click here to add your URL to Google". Voila! You are now searchable.