When using photos as wallpaper....

As I understand it, the photos need to be twice the width of the screen, even though Sense isn't scrolling across the picture on our home screens.

Therefore you want 1440 wide x 1280 high. All you'll see is the middle 720 pixels, I'd imagine.
 
You need look no further than Quickpic.
Their latest update has wallpaper support, so you can just select a portion of the picture of whatever size you want (up to a limit, of course) and set it as the wallpaper. Or if you want a dedicated app, dodol wallpaper is great also.