Simple img hover change not working. What am I missing?

Simple img hover change not working. What am I missing?

Frustrated and exhausted.  I've created a menu with the following code, but I'm getting this error (" invalid assignment left-hand side").  Please help.
  2.  <a class="nav-image" href="/menu-link'">
  3.    <img class="image-menu-rollover" src="/image.png">
  4.    <span class="nav-txt">menu-title</span>
  5. </a>

  2. jQuery.preloadImages = function() { for(var i = 0; i<arguments.length; i++) { jQuery("<img>").attr("src", arguments[i]); }}
  3. $.preloadImages('.$images.');
  4. $(function() { $("a.nav-image").hover(
  5.   function() { $(this).find("img").attr("src") = $(this).find("img").attr("src").replace("_normal","_hover"); },
  6.   function() { $(this).find("img").attr("src") = $(this).find("img").attr("src").replace("_hover","_normal"); }
  7. );})


What am I missing?  Thank you for your time.