If I understand correctly what your trying to do, there is an easy way to do this. Just measure the width that the image will need to be, and make the image that size.

I'll attach a pixel ruler for you, trust me, it comes in handy. If I misunderstood what you meant, then just call me an idiot.
P.S. - I'm pretty sure I'm allowed to attach this. If not, I apologize.