How to add alt tags with Galleria JS

I have been looking through the Galleria documentation and it seems as though adding the attribute "data-description" should generate the Alt tag, but it doesn't seem to be working.

On this documentation page, it seems as though adding an attribute of data-description to the source HTML should result in the alt tag. I have also tried leaving the alt tag in, however there is no alt tag being generated by the galleria script.

Here is the galleria script being used. The page is here: https://www.martinezcelaya.com/work.html

Galleria.run('#galleria1,#galleria2,#galleria3,#galleria4,#galleria5,#galleria6,#galleria7,#galleria8,#galleria9, #galleria10, #galleria11, #galleria12', {
   idleMode: false,
   showInfo: true,
   dataConfig: function(img) {
               return {
                   description: $(img).attr("alt")
               };
   }

   });