html_meta_tags, og:image

Hello to everyone. Sorry for my bad English.

I'm trying to use OpenGraph, but og:image doesn't work. According to source of html_meta_tags, $thumb variable is aways empty (on any pages of gallery). I'm using Zenphoto 1.4.14 [f5b47da52f] (Official build) with html_meta_tags and Graphics support: PHP GD library bundled (2.1.0 compatible). Other OG tags working perfectly.