Johannes,
I just looked at your page, and it looks like you're missing the ? in the opening < ? php tag. So, all the code is just being printed on the page, instead of the actual image.
I just looked at the page, and the code looks correct now. Facebook caches the tags, and that can take 1-3 days to refresh with the new tags you've supplied. You can use their URL debug tool to see what they see. It also has the benefit of (usually) immediately refreshing their cache as well: developers.facebook.com/tools/debug
Hope that helps, but everything does look right now.