Icons are not natively set to allow runtime images. So trying
the method you are now won't work just because by default when your
Flex application compiles it looks for that icon to embed with it.
There are two ways you can essentially get around this. A) is best
described by Ben Stucki with his post
here B) would be to use
an itemRenderer that uses an Image Control and pass the source to
that. Either one should do the trick. Hope this helps.