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=""]

http://www.ghugle.it/wp-content/uploads/2011/10/portrait-of-an-executive-working-on-computer-and-wearing-headpho1.jpg

[/auto_thumb]

[show_shortcode]

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

[/show_shortcode]

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=""]

http://www.ghugle.it/wp-content/uploads/2011/10/portrait-of-an-executive-working-on-computer-and-wearing-headpho1.jpg

[/auto_thumb]

[show_shortcode]

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

[/show_shortcode]

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"]

http://www.ghugle.it/wp-content/uploads/2011/10/portrait-of-an-executive-working-on-computer-and-wearing-headpho1.jpg

[/auto_thumb]

[show_shortcode]

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

[/show_shortcode]