Simple: Then it's not "free" as Commons understands the term and should remain on en locally. I guess that using an image that is only PD in the US is no better than using an image that is only covered by the US fair use laws, at least when we're discussing potential international reusers, which we are.
Sensible, but the portion of our images which are really free to use anywhere in the world forever may be smaller than you think. Even images created by Wikipedians and released "into the public domain" might be problematic in some jurisdictions if the image creator changes her mind about the licence. The whole concept of permanently licencing away your rights to an image is badly supported in laws around the world. Sometimes there are stipulations that any licencing of your rights to another party can only last for X years.
And I wouldn't fancy trying to defend the byzantine GFDL (which most Wikipedians have never read and Wikipedia itself is only barely in compliance with) in courtrooms around the world. And whatever the licence you'll always have to rely on the judges agreeing that by writing something into our edit boxes you're doing something legally binding.
The line dividing 'free' and 'non-free' is a lot fuzzier than one would like.
Regards, Haukur (IANAL)
P.S. Here's one issue I'm wondering about, look at this image:
http://en.wikipedia.org/wiki/Image:KingOfAtlantisEJ.jpg
This is a photo of an Icelandic sculpture. Under Icelandic laws the copyright of the person who made the sculpture is important for how the photo can be used. This image is definitely not free in Iceland. Is it free in the US? Can anyone help me find out?