From the readme...

material: skybox from dod_farmers
directory: skybox
size: 6×1024×1024
instructions: unpack the materials folder to your dod-folder (i.e. c:/programs/steam/steamapps/This email address is being protected from spambots. You need JavaScript enabled to view it./day of defeat source/dod)

source material by www.photocase.de, final skybox by pedroleum|at|gmail.com
the content of this file can be used freely in any commercial or non-commercial game. if you wish to make modifications to my work, please contact me at the above mentioned e-mail adress.