Bitmap Disappearing in Fullscreen

Hi everyone,

I am doing some work for a client, and I have just encountered a very strange problem regarding the bitmaps (.png files) used. When I view the swf movie - everything is fine until I make the browser much larger / fullscreen. Then a few of the bitmaps in the navigation menu move out of place.

Here’s a screenshot:

Can somebody please tell me how to solve this?

Many thanks in advance,