Fill up space under pages

Hello, I was wondering if there is a plug in or a conditional code to fill put the space under the pages that are not long enough to match the sidebar.

I want to place a picture or an ad, but I want wordpress to automatically re size it or chose from different pictures I assign.

Fill space http://retroquestgames.com/snesreproductions

Thank you

my site http://retroquestgames.com