After your first thumbnail, you have two /div tags. On is closing the thumbnail -- that's good, one is closing the column -- that's bad. You don't want to close your column until after the second thumbnail.
You have several more places where you have done the same sort of thing. There is some advice in here http://discuss.codecademy.com/t/5-6-7-some-advice-to-help-you-control-your-divs/9829/25 that might help show your divs that you are the boss.
The tip that might help you the most is the one about putting short comments on your /divs so that you are clear on what they are closing. Like this:
</div> <!-- closes thumbnail -->