You are missing the title attribute in your image...
<img src="_images/MU-shrubs.jpg" width="100%" title="Happy Summer!" />
Or you can set the title attribute as the Id of an element so it adds in any html...
<img src="_images/MU-shrubs.jpg" width="100%" title="#summer" /><div id="summer" class="nivo-html-caption">Happy Summer!</div>
EDIT: Both of the above should look and act the same though.
What is it you want this code to do?
Yes, that worked! Thank you! This was driving me crazy yesterday.