Seeing as WW1 was was a time that photos were pretty much black and white, you may struggle to find good original photos with what you want. I suggest looking for re-enactor websites or looking at places that sell replica military clothing.
Try this one for example. Hope it helps.....
http://www.grantsmilitaria.com/militariaphotos/gallery_default.asp?x_countryPeople=American&z_countryPeople=%3D%2C%27%2C%27
Also you may find books such as that printed by Osprey, They have a whole series of books dealing with infantry units and battles. The books usually have several pages of colour plates featuring artists illustrations of slodiers uniforms. I am sure they would have something WW1 US infantry.
Boomer...