It should probably be:
var myImage = myElement.graphics.item(0);
Does that make a difference?
Thanks Harbs, but no, that doesn't do the trick (I actually had the "0" in there, but screwed it up in my post.)
(Just a random guess:)
-- perhaps the 'parent' doesn't exist yet? Images "live" inside parent rectangles on a page, but what if it hasn't been placed yet -- does it have a parent?
After spending some more time with this today, I've realized that what Ole was talking about here: http://forums.adobe.com/message/1108974#1108974. He said that "If you're using CS4, you can use the resize method to resize graphics that have not yet been placed in a layout."
What he was referring to was graphics that are in overset text, not graphics in XML that have been imported but not placed.
So, there is no way that I know of to process images and image frames in imported XML that hasn't been placed in the layout yet.
But, a good workaround to my problem that I developed was to first place the imported XML into a very tiny temporary text frame, so that all images are immediately overset. Then I process the images to a manageable size using the resize method, per Ole's suggestion. Then, and only then, do I move the content onto my layout. This avoids problems with graphics being pointed to in the XML that may be too large for the pasteboard.
I hope this obscure technique is useful for someone!