You can't lock the wallpaper so that it does not rotate because Windows 7 resets the screen in a way that (the orientation) is more or less invisible to the applications.
You can change the way the image is streched or cropped though.
Right Click on the desktop (touch: hold down your finger until a circle appears around it.)
Click Personalize
Click Desktop Background

Select Picture position Fill, (or basically anything other than Strech) and the picture will never be distorted.
I use images that are cropped square, preferably with dimensions of the largest screen dimension, so 1200x1200 for your screen.