Add a title attribute to your img tag and it'll work. It can be empty because you're going to set it in javascript, but it has to exist. You can use a different attribute, but title is the default. http://jsfiddle.net/m2u8p/4/
<img class="1" width="50" height="50" title="" src="http://www.fancyicons.com/free-icons/112/must-have/png/256/remove_256.png">
<img class="2" title="Example 2" width="50" height="50" src="http://www.fancyicons.com/free-icons/112/must-have/png/256/remove_256.png">
$(".1").tooltip({ content : "Example"});
$(".2").tooltip();