3D FlipBook - Alt text for thumbnails?

Hi, how can I add an alt text to the thumbnail of the images? I tried by selecting a custom thumbnail image but in the front end, no alt tag is added to the img tag. I add the books via this shortcode: [3d-flip-book mode="thumbnail-lightbox" id="4825" title="true" template="black-book-view"][/3d-flip-book] Thanks