center image on feature post to homepage

Hi, 1) I would like center images on homepage : My site : http://guitare-tablatures.fr/ 2) : I have a problem with "Erreur : Missing required field "updated"." on http://www.google.com/webmasters/tools/richsnippets?q=http%3A%2F%2Fguitare-tablatures.fr%2F Have you a solution ? Thank you !