Feh

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.

Ackthets 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 &