I suspect saving them to an independent format might be the only sure solution. PDF or maybe HTML, for instance. HTML is tricky. In email, much of the graphic content may be embedded, in which case you don't have a problem, but equally, some of it may be dynamically linked, and I don't think there is any easy way to tell which it is. Perhaps taking your Thunderbird, or your whole computer offline would be the easiest way to check what is locally available.