After spending some hours of fighting with the Google Picasa API (with the help of Pedroafa), we have found that appears to be no way to get a photo URL to use for example as the src attribute of an <img> tag!
Every URI on the GData query either returns a HTTP address (for viewing the photo in Picasa Web) or the image but "pushed" so you have to download it :(
Tomorrow we will do a second round trying alternative ways, and I suppose the reason is to avoid direct linking of the images outside Picasa, but the option to link directly to the image should be available...