Making a pasted image opaque

[David Epstein]

When I use Command-control-shift-4 and drag, Mac OSX copies part of
the screen to the clipboard.  When I put that into a MC image, the
"white" part of the image is transparent.  Both for appearance's sake,
and because I want the image to receive mouseDown messages even when a
white area is clicked, I'd like to make the image I've pasted fully
"opaque".  Is there a way to script this transformation?

Many thanks.

David Epstein