There too many parameters of this shorcode. You can find detailed instructions on the documentation file. Here are some examples;

Auto thumbnail, lightbox, left align

[auto_thumb width="300" height="100" link="" lightbox="" align="left" title="" alt="" iframe=""] full path of your image [/auto_thumb]

Auto thumbnail, link to a video, lightbox, right align

[auto_thumb width="300" height="100" link="http://vimeo.com/8245346" lightbox="" align="right" title="" alt="" iframe=""] full path of your image [/auto_thumb]

Auto thumbnail, link to an external url, lightbox, center align

[auto_thumb width="300" height="100" link="http://google.com" lightbox="" align="center" title="" alt="" iframe="true"] full path of your image [/auto_thumb]