Swapping Images Causes Blanks....

I know this is sorta reposting but I want to simplify the issue a bit. Take the following script:

function replacemain(s){
var x = document.images['MainImage'];
var y = document.images[s];
var pictemp = x.src;
x.src = y.src;
y.src = pictemp;
}

All I want to do is replace the image specified as "s" with the MainImage. Actually swap them so that each execution of the function will swapp and reswap the images without loosing anything. This generally works, however, on a couple IE6 browsers it causes blank images to swap in....