Freshmeat is right.

And a good way to position your billboard is to move it relitative to the camera. This will allow you to have it centered and not tilted.


Steps after importing:

Adjust the Billbord to the position you want, using "orient to face" You may want to go to the "add objects" menu to resize the picture for the best fit.

Now Under the properties tab of your billboard find vehicle

Set vehicle to camera.

When your done with the billboard just select "is Showing" = false