Hi, had the same problem; not sure which launcher you using.
Part 1) If using Zeam (I am) goto workspace preferences and unselect scroll wallpaper.
Part 2) I use JustPictures by Kounch (free). Go to where your pictures are displayed as thumbnails. Click and hold on the picture you want as the background, and select set as wallpaper.
The image will not be cropped, and because of the zeam settings not stretched over all screens i.e. each homescreen will have the same picture fullscreen.