Hi Skywalker,

1) connect as an admin
2) in the gallery, go to the picture.php page with the photo you want as representative
3) click on the "modify information" icon
4) you are on the administration screen for the given photo, go to the "Categories representation" box (bottom of the page)
5) select your category in the "Does not represent" list and click on the "<<" button to make it represents your category

You're done :-)

I have a question as I could, to automatically make the first image was taken in alphabetical order.