From http://alilknowhow.com/2011/01/09/worki ... hare-link/
"If you have fixed everything and it still doesn’t show, that is because you have already attempted to share the link previously, and facebook caches the thumbnails server side when you share a link. So if you share it again, rather than download the new thumbnail, facebook calls up it’s saved thumbnail from its own cache.Solution
: Share the same link but slightly different. [...] If you are just using plain old HTML, try directing straight to your index page by changing the link to http://example.com/index.html
rather than http://example.com/
You can apparently also go to https://developers.facebook.com/tools/debug
to clear your old thumbnail from Facebook's cache instead of changing the URL.