Mouse Over Image

I am creating a collage of Pictures… I have all the pictures as small thumbnails scattered across the page. each is created as a button. I want the user to be able to mouse over each small thumbnail, and the image to appear larger on the screen, not anywhere in particular just on top of all the other thmbnails. I can create the larger image but when I put the larger image in the mouse over keyframe of the button then test it, the hidden larger image acts as the activation as well to the larger image… HELP!!!