More detail on binary to XPCOM

HI

I would like to play with the data inside a image loaded from the 
network.  an example is
<img src="foo.gif"><img src="bar.png">

document.images[0] is foo.gif
document.images[1] is bar.png

I then want to see the data for the images

in idl file

interface xxxxx : nsISupports
{

     string checkIt( in ????? *img);    ***** question 1
     newimg makeIt(in ????? *oldimg );  ***** question 3
}

so from JS
    var str;
    str = comp.checkIt(document.images[i]);
    var oldimg = new Image();
    var newimg = makeIt( oldimg );



then from the XPCOM
NS?IMP    xxxxx::CheckIt(?????? * img, char * _retval)
{

       realimg = getreal( img )    ***** question 2
       switch (realimg.type ) {
       case PNG:
            pixelmap = png2pixmap(realimg);

            break;
       case GIF:
            pixelmap = gif2pixmap(realimg);
            break;
       default:
       }

       *_retval = checkpixmap(pixelmap)
}

1>  First I need to know how to setup the idl file to accept 
(nsIDOMHTMLImageElement), if that is the right type ???

2>  next I need to know how to get to the data from the ImageElement.
	I am assuming the type passed in is only a handle for the image, 	and I 
need to get the real image out of cache.


3>  The last thing is how do I return a Image from XPCOM

Shaun