This is what you wanna do:
var oldSrc="http://example.com/smith.gif";
var newSrc="http://example.com/johnson.gif";
$('img[src="' + oldSrc + '"]').attr('src', newSrc);
This is what you wanna do:
var oldSrc="http://example.com/smith.gif";
var newSrc="http://example.com/johnson.gif";
$('img[src="' + oldSrc + '"]').attr('src', newSrc);