Problem thumb

Hi, I have a problem with the thumbnail images for each item!

In my template, the thumb is drawn through each item … but they must be set manually, so do not automatically take the first image loaded.

I would like to see the thumb automatically, but I can not do this.
And ‘course to edit the page thumb.php … but do not know what to change.

Help me please, I can send or write any information you need to see a preview of the site: http://www.gratis4ever.com

Thank you.