Bug 104986 Backgound image uses an absolute path
Currently Chart uses an absolute path to indicate a backgournd image, that may cause the design unusable on other environments. Expected the background image can be definded in multiple ways like in report designer. e.g. URI(absolute path), File(relative path), Embedded Image(Encoded Image Data).
chart/org.eclipse.birt.chart.engine/src/org/eclipse/birt/chart/model/attribute/AttributeFactory.java
* Returns a new object of class '<em>Action Value</em>'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return a new object of class '<em>Action Value</em>'.
	 * @generated
	 */
	ActionValue createActionValue( );
	/**
	 * Returns a new object of class '<em>Angle3 D</em>'.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @return a new object of class '<em>Angle3 D</em>'.
	 * @generated
	 */
	Angle3D createAngle3D( );
	/**
	 * Returns a new object of class '<em>Axis Origin</em>'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return a new object of class '<em>Axis Origin</em>'.
	 * @generated
	 */
	AxisOrigin createAxisOrigin( );
	/**
	 * Returns a new object of class '<em>Bounds</em>'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return a new object of class '<em>Bounds</em>'.
	 * @generated
	 */
	Bounds createBounds( );
	/**
	 * Returns a new object of class '<em>Call Back Value</em>'.