I think I understand what you are wanting. To do this, try
creating an image that is sized identically to your project (or
rollover slidelet area if that's what you are using). I would use
Microsoft Paint to do this, but you may use any image editor you
like. Just create a large image with the area you wish the image to
show in the center. Color the outside rectangle as you like, but
use a specific solid color in the center. Then insert that image
into your slide (or slidelet) in front of everything else.
Configure the center color as transparent.