I just found out that the problem might be the skyboxfile I am using since I figured out that it doesn't work for any map I go on.
I am trying to get the skybox which you can find in mp_brony, mp_on_the_sky and mp_sunset in mp_the_extreme.
I've been told that pripyat_ft is the name of that skybox, I found that file in 'localized_english_aw03' in the 'main' folder and made 50 copies of it in a new 'images' folder, then renamed every file by all kinds of different names of other 'cod4 original' skyboxes and once I did that I made the 'images' folder in a zipfile.
Then I put that file in the folder of '3xp_cj', started cod4, went on devmap, but saw no skies have been changed.
Something else I checked out was the iwd. file of mp_the_extreme which has quite a lot of imagefiles included, I turned all those files into DDS files, looked at them all but didn't find any image that looked like a skybox, so there isn't a skybox in the mp_the_extreme file either which I could replace.
Anyway, if someone could manage to figure out a way or manage to get a working skyskin of the skybox you can find on mp_brony, mp_on_the_sky and mp_sunset please send me a message here