Copy Source_image Src To Another Image From Onclick

Im having trouble with my js. I have a series of img tags. each one needs to run a function with the onclick event. In the function I want to copy the src of the image into the src of a main -larger- img tag. I tried something like this:
function doit(){

var node = this;
var dest_div = document.getElementById("bigimagediv");

dest_div.replaceNode(node, dest_img);
node.onclick = null;
}

I tried this but its 'not a function' =/
function doit(){
var node = this;
var nodesrc = node.getAttribute("src");
dest_img.setAttribute("src", nodesrc);
}

but it doesnt work. I _do_ want to the keep the image thumbnail on the page, so maybe a copyNode or something else.