Take a look at the graphics part of the GoogleAPI reference. It says that loadImage takes either a string, or the responseStream property of an XMLHttp. By XMLHttp, I assume it means XMLHttpRequest. 
My problem is that I can't find documentation for this field (the responseStream field) anywhere. I've had no luck googling it (ironc), it's not in "JavaScript: The Definitive Guide"... yet these gadgets that use it seem to work.
This post says that responseStream seems to not be accessible from a browser's XMLHttpRequest. From whose is it, then? Is it just a Google Desktop thing?