Well, this problem is because the texture, used for the skybox is not a power of 2. For example if your texture is 600 x 600 it won't work, I had the same problem almost an year ago. So, you just have to resize the skybox textures to something like 512 x 512 or 1024 x 1024 or 256x 256 (I don't recommend that size for cool graphics

)