OK! we're close... If I attach this to a variable and echo it out, it give's me the correct string. The _off is replaced with _over and the other way around. As I have it below, on hover, the image will not change, no errors (firebug), just doesn't do anything.
$('li.clickable').hover(function() { $(this).find('img').attr('src').replace("_off","_over"); }, function() { $(this).find('img').attr('src').replace("_over","_off"); });