<record>
<!-- resource manager options -->
<!-- dont preload anything in this folder -->
<boolean id="preload" value="false"/>
<!-- turn off auto mapping -->
<boolean id="amap" value="false"/>
<!-- background mappings -->
<!-- the following XML maps pictures inside this folder into other positions
in the resource system, which allows this folder to be dropped into any
place in the graphics folder and still have the game pick up the graphics
files from the correct places
-->
<list id="maps">
<!-- Auto generated by fmXML -->
<record from="inter" to="graphics/pictures/team/1135/background"/>
</list>
</record>
The pictures you want as the background for the clubs go into the data/graphics/backgrounds/clubs folder with above XML as an example. The name of the team "inter" is the name of the background file and the number "1135" is the unique ID from inside the game.
If you want to add more just use the same method.