For some reason this script will work with all of the 'echo's at the end, but without them $wall is an empty string. This seems like really odd behaviour.

#!/bin/zsh

if [ ! -n "$1" ] ; then
    files=(~/pictures/backgrounds/*jpg)
else
    while [ $1 ] ; do
        files+=(`echo $1/*jpg`)
        shift
    done
fi
echo $files
N=${#files}
echo $N
((N=RANDOM%N))
echo $N
wall=${files[$N]}
echo $wall
cp $wall ~/wall.jpg