I am using flash to make a slideshow of images.  4-5 images that fade, on into the other.  I would like to make it so that the corners, of the entire slideshow, are rounded and transparent.  I want to do this on the slideshow level as opposed to just editing the images themselves so that this slideshow can serve as a template where any images can be dropped into it.

The person that worked hat this position before me made some templates with rounded corners, but I cannot figure out how to duplicate how he did it.  As near as I can tell, he merely made a layer on top of all of the other layers and filled in the area he wanted to be transparent with white, but when I do the same it doesn't work, so there must be a step I am missing.

Thoughts?