Wallpaper Generating Script works but not when run as a cron job

Question

I wrote a python script to autogenerate a background pattern for my computer. I want to use cron to run it every 15 minutes or so. Here's the code:

#!/usr/bin/python
import os
import numpy as np
import svgwrite as svg

WIDTH = 1600
HEIGHT = 900

def circle_layer(dwg, size_range, blur, number):

    blur_filter = dwg.defs.add(dwg.filter())
    blur_filter.feGaussianBlur(in_='SourceGraphic', stdDeviation=blur)
    g_f = dwg.add(dwg.g(filter=blur_filter.get_funciri()))

    for c in xrange(0, number):
        (x, y) = (np.random.random_integers(0,WIDTH),    np.random.random_integers(0,HEIGHT))
        rad = np.random.random_integers(size_range[0], size_range[1])
        g_f.add(dwg.circle(center=(str(x),str(y)), r=str(rad),stroke="rgb(100,100,100)", fill="rgb(100,0,0)"))

    return dwg




if __name__ == "__main__":

    svg_doc = svg.Drawing(filename = "current_wallpaper.svg", size = (str(WIDTH), str(HEIGHT)))

    svg_doc.add(svg_doc.rect(insert=(0,0), size=(str(WIDTH), str(HEIGHT)), fill="rgb(0,0,0)"))


    svg_doc = circle_layer(svg_doc, (10,50),  20, 100)
    svg_doc = circle_layer(svg_doc, (10, 20), 4, 100)

    svg_doc.save()

    import subprocess

    proc = subprocess.Popen(["pgrep gnome-session"], stdout=subprocess.PIPE, shell=True)
    (pid, err) = proc.communicate()
    pid = pid.rstrip('\t\r\n\0')
    print repr(pid)

    cmd = "grep -z DBUS_SESSION_BUS_ADDRESS /proc/"+pid+"/environ|cut -d= -f2-"
    proc = subprocess.Popen([cmd], stdout=subprocess.PIPE, shell=True)
    (out, err) = proc.communicate()
    out = out.rstrip('\t\r\n\0')
    print repr(out)


    cmd = "export DBUS_SESSION_BUS_ADDRESS=$"+out
    print repr(cmd)
    os.system(cmd)

    os.system("/usr/bin/gsettings set org.gnome.desktop.background picture-uri file:///home/jofo/Pictures/Wallpapers/current_wallpaper.svg")
    print "I ran!"

Basically, it uses svgwrite to draw some circles and add a gaussian blur. Then, I make a system call to gsettings to update the ubuntu wallpaper.

This works fine when I run it as ./wallpaper.py, but when I schedule it in crontab, the script runs, and the background pattern doesn't change.

Here's what I have in my crontab file doohickey:

MAILTO=""
*/1 * * * * /home/jofo/Pictures/Wallpapers/./wallpaper.py >> /home/jofo/Pictures/Wallpapers/cron.log

I know the script is running once a minute because it appends "I ran!" to the cron.log file once per minute. Unfortunately, the wallpaper never changes. If anyone knows why this might be, that would be great!