Replace Image Source With Other Source

I have a website that I frequently visit (FWIW, Firefox 3.x is my browser of choice) with many image sources referring to URLs that end with "-thumbnail.jpg". However, for better image quality, I am trying to use Greasemonkey to replace all instances of "-thumbnail.jpg" in the source of images on this site with "-bigthumbnail.jpg". The closest I could think of was to somehow use getElementsByTagName and innerHTML.replace, but realized that innerHTML does not do HTML, only content.

Below is as far as I tried to get on my own,

var as,ae;
as = document.getElementsByTagName("img");
for (var i = 0; i < as.length; i++) {
ae = as[i];
ae.innerHTML = ae.innerHTML.replace(/-thumbnail/gi, "-bigthumbnail");
}
})();