button rollover

hello all,
this is probably a really simple question... it's been awhile since i've done something like this, and my brain is failing me.

i'm creating a image gallery in which the buttons are thumbnails of the larger images. the idea is, you rollover the thumbnail button to show the large image. however, the large images are not only showing up on the rollover of the button, but also when you rollover the area where the larger image will ultimately be. that's not what i want to happen!

basically i'm placing the large image in the over and down states of the button. apparently that's not the thing to do?

i was hoping to have this done tonight...
any help/suggestions greatly appreciated!

x-posted to flashdesigners