Allow SHA1 for image names for now: they're admin-provided
    
    Once non-admins can upload images, we should look at migrating over
    to another hash.
    
    Signed-off-by: Patrick Uiterwijk <patrick@puiterwijk.org>
    Reviewed-by: Randy Barlow <randy@electronsweatshop.com>
    Reviewed-by: Michal Konečný <michal.konecny@packetseekers.eu>