Thanks Craige@JMP. That works.
One complication is that if I replace the <type> argument with a column reference, it seems to no longer work. Do you know why?
Names Default To Here( 1 );
dt = New Table( "Image Test",
Add Rows( 1 ),
New Column( "Image Type", Character, Nominal, Set Values( {"png"} ) ),
New Column( "Image from URL",
Expression,
Formula(
New Image( Open( "http://eits.uga.edu/_resources/files/images/JMP_logo.png", :Image Type ) )
)
),
New Column( "Local Image",
Expression,
Formula( New Image( Open( "$desktop/circle.png", :Image Type ) ) )
)
);