Try this,
<mx:canvas creationComplete="init()">
<mx:Script> <![CDATA[
import mx.controls.Alert;
[Bindable]
[Embed(source="/assets/graphics/arrowRight.jpg")]
public var myIcon1:Class;
[Embed(source="/assets/graphics/btn_done_off.gif")]
public var myIcon2:Class;
[Embed(source="/assets/graphics/btn_exit_off.gif")]
public var myIcon3:Class;
[Embed(source="/assets/graphics/arrowRightBig.jpg")]
public var myIcon4:Class;
[Embed(source="/assets/graphics/question2.gif")]
public var myIcon5:Class;
private function init():void {
var tab:Tab;
var len:uint = tabBar.numChildren;
for (i=0; i<len; i++) {
tab = tabBar.getChildAt(i) as Tab;
tab.labelPlacement = "left";
}
]]>
</mx:Script>
<mx:Spacer height="1000" >
</mx:Spacer>
<mx:TabNavigator id="tabBar" width="50%" height="50%" >
<mx:VBox id="one" label="one" icon="{myIcon1}" click="Alert.show('Tab1');" />
<mx:VBox id="two" label="two" icon="{myIcon2}" click="Alert.show('Tab2');" />
<mx:VBox id="three" label="three" icon="{myIcon3}" click="Alert.show('Tab3');" />
<mx:VBox id="four" label="four" icon="{myIcon4}" click="Alert.show('Tab4');" />
<mx:VBox id="five" label="five" icon="{myIcon5}" click="Alert.show('Tab5');" />
</mx:TabNavigator>
</mx:canvas>
Hope it helps.