You need to assign them to have them appear in your game.
I would suggest doing a search as I remember seeing someone else ask this.
I know you can assign pictures to individuals but haven't tried doing this randomly.
At a guess, I would suggest you need to edit the config.xml file in the \data\graphics\backgrounds folder where you have placed the pictures.
This will look like this:-
<record>
<!-- resource manager options -->
<!-- dont preload or cache anything in this folder -->
<boolean id="preload" value="false"/>
<boolean id="cache" value="false"/>
<!-- turn on auto mapping -->
<boolean id="amap" value="true"/>
<list id="maps">
<record from="default" to="graphics/backgrounds/default/default"/>
</list>
</record>
I would suggest adding a line of code for each picture you would like to randomly appear.
The line you would be intersted in would be
<record from="default" to="graphics/backgrounds/default/default"/>
You would need to change the word default to whatever your picture is called.
For example, if your picture is called ronaldo.jpg you need to add the line
<record from="ronaldo" to="graphics/backgrounds/default/default"/>
Your file would then look like the below.
<record>
<!-- resource manager options -->
<!-- dont preload or cache anything in this folder -->
<boolean id="preload" value="false"/>
<boolean id="cache" value="false"/>
<!-- turn on auto mapping -->
<boolean id="amap" value="true"/>
<list id="maps">
<record from="default" to="graphics/backgrounds/default/default"/>
<record from="ronaldo" to="graphics/backgrounds/default/default"/>
</list>
</record>
Hope this helps.