As far as I can tell, the Alt text is working correctly. For example, look at this generated link from my test site:
<img class=”atw-gallery-img” src=”http://example.com/wp-content/uploads/2011/07/dsc20050102_192118_51.jpg” alt=”Wind Farm Alt Text” title=”Wind Farm” />
In this case, the Alt and Title text come from the corresponding entries in the Media Library for the image: If you open an image in the Media Library, you should see a bunch of options on the right side, including URL, Title, Caption, Alt Text, Description, and some other information. That is the information that is used to fill in the Alt and Title options. This is for Show Sliders Version 1.2.3.