feh is an image viewer but it is mainly used for setting background images on minimalistic desktop setups. To get it to set your background when X starts, put the line
feh --bg-scale /path/to/image
in your .xinitrc.
Ackthet's feh script
The following script changes your background to a random image in the folder ~/.bg every 15 minutes. It should be called in your .xinitrc so that its started with X.
#!/bin/sh while true; do find $HOME/.bg -type f -name '*.jpg' -o -name '*.png' | shuf -n 1 | xargs feh --bg-scale sleep 15m done &