interior and exterior colors of a map

Hi useR's,
I have an image plot I want to overlay a map of the United States on, via
map('usa').  The image is basically a large rectangular of various colors. 
When I overlay the United States map, the full rectangle of the image plot
is visible, but I only want to display the image WITHIN the United States
boundary.
Does anyone know a way to do this?
Thanks in advance,

dxc13

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