Well those rifles they are carrying look as if they're trying to be G98s (though they don't look quite right to me), which would mean 1897ish onwards. So Boxer Rebellion and African Colonial business (German Southwest 1904-1908: Herero and Nama revolts, German East 1905-1906: Maji-Maji Revolt).
They fought in the Great War, but I'm not aware of them wearing that particular tropical/summer uniform while doing so.