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