Bug 1068967 - Make thumbnails accessible.

I don't think (but I could be wrong) that we've waited for l10n.ready() here, so this call to get() may happen before the locale db is loaded. Basically we've got a race condition.  And then there is the related problem of re-localizaing the alt strings if the user switches locales.

See bug 1059682 for another bug that involves dynamic relocalization in the gallery. Let's land that first, then integrate the alt tag setting of this patch with the framework from that bug.