Need To Change Src Attr Of A Destination Image From One This Clicked On

Inside of an img tag I have an onclick that fires a function. The
function is supposed to change the src of a larger image elsewhere on
the page. Think: click the thumbnail to see it at the enlarged_image
that is on the page.

I tried this function but it doesnt work =/

function doit(){
var node = this;
var nodesrc = node.getAttribute("src");
dest_img.setAttribute("src", nodesrc);
}

I dont think 'this' is doing what I want it to, plus I get the error
that node.getAttribute() is not a function.