|*the photo hosting site for which I posted this tip now has the option of sharing photos by URL only; most others have the option of making photos 'public' -- but I thought I'd leave it up, just in case there are still photo hosts that don't share photos easily.*
The problems are from trying to allow people who are not you view your photos. If the site allows you to 'share photos' - that is, email a link to friends so they can view your pictures -- then you can email the photo link to yourself, click through on the email to the photo, copy that link and paste it into your review. It's kind of complicated, but since the photo posting site considers that a shared link it should bypass the login pages on those sites.
Hope that helps someone!