Try to use this way:

JavaScript

img1.alt="Image alt";
img1.title="Image title";

Thumbnail HTML:

<a href="javascript:document['mainimage'].src = img1.src;document['mainimage'].alt = img1.alt; document['mainimage'].title = img1.title; javascript:void(0);"><img src="{$smarty.const.dir_images}/t_{$this_page.image1}" title="" alt=""/></a>