edit /usr/share/backgrounds/beefy-miracle/default/beefy-miracle.xml,change the picture path which you set as below:
<!-- Wide 16:10 -->
<size width="1920" height="1200">/usr/share/backgrounds/beefy-miracle/default/wide/SundownDunes.jpg</size>
<!-- Standard 4:3 -->
<size width="2048" height="1536">/usr/share/backgrounds/beefy-miracle/default/standard/SundownDunes.jpg</size>
<!-- Normalish 5:4 -->
<size width="1280" height="1024">/usr/share/backgrounds/beefy-miracle/default/normalish/SundownDunes.jpg</size>
See http://fabhax.com/technology/change-wallpapers-in-gnome-3.4/ for a step by step instruction on how to change each of the three wallpapers on GNOME 3. This article is written for Debian though.
– Susam Pal – 2012-10-16T04:24:57.167