Regularly updating your wallpaper (on Gnome)

You can automatically update your wallpaper on a regular basis by having a cron job calling a bash script.
The bash script is very simple: it runs with your parameters and tells Gnome to update the wallpaper. Copy this file and edit it to suit your preferences:

# update the wallpaper in Gnome
# Koant (
# If it doesn't work, see for an alternative

OUTPUTPATH='/home/XXXX/temp' # path of directory where you want the image to be stored. 
OUTPUT='mylastfmwallpaper' # filename of the wallpaper. Do not add .jpg

# run with your options. But don't change the filename.

#update Gnome background
/usr/bin/gconftool -t str -s /desktop/gnome/background/picture_filename "$OUTPUTPATH/$OUTPUT.jpg"

Next, make it executable (chmod a+x update_wallpaper) and set up a cron job to have it run at regular times (crontab -e).
For example:

Note that:
1. Charts are updated once week, so more frequent updates probably aren't necessary.
2. Please be kind to the servers and don't update more than once a day. Use the '-l' or '--Local' switch to use a local copy of the charts - you can even use it offline!

Using the scripts for artists' pictures

Roignac has extended the script for using artists' pictures. You can find his version here.

Background colour and transparency

HelgeBS added options for choosing the background colour (default is black) or even set it to transparent. His code can be found here.

Updating your wallpaper on Windows XP/Vista/7 in one click (or at startup)

Thrar has a way to update your wallpaper on windows XP in one click, instead of going through the whole desktop/properties/etc. The script can also be used at startup.
DesertBlade also suggested a program for setting-up the wallpaper in Vista/Window 7. It can be used for updating it regularly as well.

Issue on OSX: "decoder jpeg not available"

Some OSX users might come across this error message when running Apparently, it's a common problem on OSX but a fix can be found here: Thanks to zephirnl for pointing out the issue.

Path on OSX

Python might be installed somewhere else in OSX than the location specified in the script. Find where Python is installed with: which python and replace the first line of the script by the correct path. Thanks to m00nt00n for noticing this.