This is probably very old news, but since I did not find any post about this, i am posting here.
I am i need of rotate the bg images, something that is lacking in Blender. You have the possibility to change the size and moving it up and down, and left and right. So this was the solution for me:
Step 1: Create an empty (choose type image)
Step 2: Open the desired image
Thats it! now you have full control
Esim:)