small change to the code

On line 17, you have:
if(!$atts['size'])
size might not be defined which triggers a warning telling you that your using an invalid offset. It should be replaced by:
if(empty($atts['size']))
which checks if it’s set first.

Thanks for the plugin.

http://ww.wp.xz.cn/extend/plugins/add-post-thumbnail-shortcode/