You can use the Rotation setting in the material editor, but when it's a matter of just needing to go from vertical to horizontal, I've found it's a lot easier to just rotate the image itself in a paint program and save it as a new file. Then I've got one marked H(orizontal) and one marked V(ertical), and I can just grab whichever will work best for the primitive I'm using at the time. :)