- From File, select Save as...
- Use whatever file name and location you want, and from the pop-up, pick "Raw message source".
- Go to the Finder and drag the file you just saved to Suffit-Expander (which I have on the Dock)
[Editor's note: Sent as an anonymous hint, you can also "pretend you're going to forward the images to someone (by hitting the forward button) and then either drag the icon to the desktop, or click to open in your default image program." I haven't tried this myself, but it may well work.]

