#!/bin/bash
    while true ; do
    osascript <<EOF
        tell application "System Events"
            tell every desktop
                set picture to "/Users/bill/Desktop/bckg_changer/ARM_1885.jpg"
            end tell
        end tell
    EOF
done

Bills-iMac-4:test bill$ ./test.sh
./test.sh: line 11: syntax error: unexpected end of file