Are you absolutely sure that it is PD in the US? Only
if you can be
sure that it was first published/copyright registered in the US. If
it was created or first published elsewhere, then the pre-1923 rule
does not apply, thanks to international convention.
Certainly the image description page does not answer this question,
but it mentions a book first published in London, which would count
against this theory if that was the first publication of this image.
*sigh* I honestly thought I was getting the hang of this image licencing
business but there's always more to it... Thanks for the pointer, I'll try
to find out.
Regards,
Haukur