Because you're inside a mx:Component tag, your scope has changed: this now refers to the itemRenderer component.
You can resolve to the larger scope by using outerDocument. The event handler function does need to be public since it's being called from another class.
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" layout="absolute">
<mx:Script>
<![CDATA[
public function onClick(event:Event):void {}
]]>
</mx:Script>
<mx:ComboBox>
<mx:itemRenderer>
<mx:Component>
<mx:Image click="{outerDocument.onClick(event)}" />
</mx:Component>
</mx:itemRenderer>
</mx:ComboBox>
</mx:Application>