Two of my great grandfathers served in WWI. My grandpa was able to tell me quite a bit about his dad, but no one is left who remembers the military details about my other great grandfather.
The photos can be found here: http://imgur.com/gallery/7lvc5
The uniform in the top photo was taken in 1918 and I am pretty sure is American Infantryman. The bottom photo was taken in 1917 and I'm having trouble identifying that uniform.
The top one (with hat) does appear to be an American Infantryman. I suspect he was a relatively new recruit, as he is missing rank and organizational insignia. Additionally, the uniform does not appear too worn, and the leggings are almost pristine. I guess it is possible that he was a US Marine, but I do not see an EGA (Eagle, Globe, Anchor) on the hat, as I would expect. The bottom one is definitely US Army-- He served in "K" Company of the 160th Infantry Regiment.
Hello there! As your question is related to looking for identification/information regarding military personnel, our Guide on Military Identification may be of use to you. It provides a number of different resources, including how to request service records from a number of national agencies around the world, as well as graphical aids to assist in deciphering rank, unit, and other forms of badges or insignia. While the users here may still be able to lend you more assistance, hopefully this will provide a good place to start!