Drawables position in GenericPicker

I'm trying to develop a simple text input GUI for touch based devices. I extended the PickerFactory to return custom symbols in a GenericPicker:

function getDrawable(index, selected) {
    return new Rez.Drawables.pickerBtnDone();
}

In my resources.xml I have this:

<drawable-list id="pickerBtnDone" background="Graphics.COLOR_TRANSPARENT">
	<bitmap id="iconDone" filename="icons/picker_key_done.png" />
</drawable-list>

The symbol is displayed like this:

The bitmap is a 48x48 pixel icon. If I set x="center" y="center" in the bitmap properties:

Is there some guideline to implement the picker with icons from resources?