OK,  I am assuming you are using a version of DNN that includes the jQuery library.  If so,  when you add the events module to a page.  Add the following code in the modules settings -> Advanced Settings -> Header box:  (of course you will need to make sure the image src is correct and change the alt text to what you want.  that should do it.  Another example of jQuery to the rescue.

<script>
jQuery(document).ready(function() {
     jQuery("img[src='/dnn494_NBStore/images/progressbar.gif']").attr("alt", 'Alt title goes here');       
});
</script>