Twitterbridge doesn't update avatars if if was manually deleted. Last message in logs about it:
2014-05-25 13:15:52 LOG_DEBUG: [sn.pztrn.name:queuedaemon.php:3827] TwitterImport::updateAvatar - No avatar found for (profile_id=1841)
And after it twitterbridge doesn't even trying to update avatar or redownload it, if missing.