I wrote:

[...]
> But with `xpm-create-image' hooked into `find-image' as a fall-back
> for XPMs the image descriptor looks like this:
>
> (image :data "P6 32 32 255 ... <binary data> ..." :type pbm ... )
[...]

I guess, I should show my code. Attached is a patch to image.el. But I
would like to test it more extensively.