It means you have to install the hi res pack. Without this pack you are not running 1024 resolution textures.
After installing the pack, you must set 1024 in your graphics settings, then export the skin. When working on the skin, also do so in 1024 to preview work. Otherwise you may reduce this setting for actual gameplay.
The skin, however, still doesn't belong in the game. Same as with the D.520 wish thread, the only argument for it is "it's French" -- not that it was relevant.