Why do pictures sometimes not show as a thumbnail view and are maximized in the post?? How can I correct this??

I've successfully up-loaded several pictures in the past and use the method in the "how to post pictures" section...They almost always show as a small thumbnail...This large picture thing doesn't happen to me that much but I have seen other guys posts...Especially in the classic pattern forums which I frequent..It appears to be a random thing and the poster has no control over it...

P.S. Just like this!! Yay it's doing it so I can show you...I wanted this picture to be a small thumbnail and not full size in this post...