Sam, changing the dimensions of the image--12 x 60 or 60 x 12--doesn't change the direction of the wood grain. It does distort the grain a bit.
To rotate the material, though, you would right click on the face after you've applied the material, choose Texture>Position. right click on the face again and choose Rotate>90°. If you've applied the material to a group or component instead of to faces, you won't get the Texture entry in the Context menu. If you rotate the texture this way, you can get away with a single version of the material in the model. Some people like to make vertical and horizontal versions of their wood grain materials. This to me seems wasteful and unnecessary.
You might find some ideas
here.