So, this is my try:
// Load textures from the mod folder
posX = SpaceWarp.API.Assets.AssetManager.GetAsset<Texture2D>("MODID/images/GalaxyTex_PositiveY.dds");
posY = SpaceWarp.API.Assets.AssetManager.GetAsset<Texture2D>("MODID/images/GalaxyTex_PositiveY.dds");
posZ = SpaceWarp.API.Assets.AssetManager.GetAsset<Texture2D>("MODID/images/GalaxyTex_PositiveZ.dds");
negX = SpaceWarp.API.Assets.AssetManager.GetAsset<Texture2D>("MODID/images/GalaxyTex_NegativeX.dds");
negY = SpaceWarp.API.Assets.AssetManager.GetAsset<Texture2D>("MODID/images/GalaxyTex_NegativeY.dds");
negZ = SpaceWarp.API.Assets.AssetManager.GetAsset<Texture2D>("MODID/images/GalaxyTex_NegativeZ.dds");