of course... of course once I post it I remember... so that's how it's done for others to learn:
1.) Create an empty Sphere ("Game Object" -> "3D Object" -> "Sphere"), then pump it up to lets say 150m x 150m x 150m and set it to X,Y,Z = 0,0,0.
2.) Drag your 3D / Skybox picture onto the sphere...