Re: XPM via Lisp in the toolbar

[Richard Stallman]

    Or I could hook `xpm-create-image' into the appropriate functions in
    image.el.

This seems like the cleanest result.  If Emacs does not support XPM
format internally, it can use your package.  So XPM will always "just
work" as far as the user is concerned.

    But what to do with
    the tool bar? On MS Windows this breaks the tool bar totally. 

Could you explain why it has that effect?  That is not obvious.  It is
surprising that adding XPM support via conversion should break
anything that worked previously.

    Or I could add a caching mechanism to toolbar.el so that the
    conversion happens only once.

Why not add the caching feature in your conversion library?
Then it would apply to anything that tries to display XPM
and works via conversion.