Solved! Problem was my sky color wasn't transparent... I had a feeling setting a negative layer number would be it but it kept disapearing... Because of sky color.
Code:
ENTITY* skycubepic =
{
layer = -1;
x = 100;
y = 0;
z = 0;
type = "cosmo_f02+6.bmp"; // SET BY USER
flags2 = SKY | SHOW ;
Code:
function main()
{
...
vec_set(sky_color.blue,vector(0,0,0));
...
}