For loopable: Crop an image to 3200x480 . i'm using photoscape - crop/assign ratio/size - for that, (use your imagination to "see" how will look like). put it on your mass memory. Install WP Creator (wallpaper creator) on your N900 (from app. manager/faster app manager). WPcreator will make all the work, it makes a set too.
For 4 different walls: install app "wallpaper" or "Live wallpaper". Live wallpaper have also slideshow.