Either, use VertexTexCoords command to adjust the texture coords for the cube (that means, you just use the one texture.
Or, create a cube in a modelling program, paint it, then export into blitz.
Or, create the cube from quad, and paint each quad.
Or, create a cube in a modelling program, paint it, then export into blitz.
Or, create the cube from quad, and paint each quad.