Thank you for your kind word, Pete. Actually, this slogan has been reviewed by /two/ native speakers, one from the United States, and the other from the United Kingdom.
I also cannot see a reason for the US to have a different banner from the rest of the world, as we are /not/ using a cultural heritage-related term ("monuments" vs "historic sites") this time.
As for the banners, we are currently using two different ones on all projects: one for all participating countries except Israel ("Wiki Loves Monuments: Photograph a monument, help Wikipedia, and win!"), and one only for the US ("Wiki Loves Monuments: Historic sites, photos, and prizes!").