The below pictures of Mongolia are free, they are Creative Commons CC0 (CC0 1.0 Universal (CC0 1.0) Public Domain Dedication). please note that these photos have been tagged with "Mongolia" so there maybe some errors as I have not checked them all. On each image page I have provided a link to the source of the photo though.
Click on the picture to see a larger version or click on the number to see more info about the Mongolia picture and a link to it's source where you can download a higher resolution photo.