Make thumbnail appear when pasting gallery link

jnrpotography

I don't post actual photos on facebook, but I do post links to galleries there via my fan page. My problem is that when I copy and past some links there I have an option to choose a photo from the gallery as a teaser preview thumbnail and it only appears for some of the links I copy and paste. Why does this happen? I would like a photo from gallery as teaser preview everytime I copy and paste a link. How can I get this to happen? Please click link to know what I an talking about. Please look at the last link I posted - DeLaBuena. There is no thumbnail because I didn't get photos as options when I pasted it, but if you scroll down you will see that with every other gallery link I posted I have a thumbnail from that gallery. http://www.facebook.com/#/pages/JNR-Photography/183695276681

Help!:scratch