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/AttributePackage.java
* <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	int STYLE__BACKGROUND_IMAGE = 3;
	/**
	 * The feature id for the '<em><b>Padding</b></em>' containment reference.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	int STYLE__PADDING = 4;
	/**
	 * The number of structural features of the the '<em>Style</em>' class.
	 * <!-- begin-user-doc -->
	 * <!-- end-user-doc -->
	 * @generated
	 * @ordered
	 */
	int STYLE_FEATURE_COUNT = 5;
	/**
	 * The meta object id for the '{@link org.eclipse.birt.chart.model.attribute.impl.StyleMapImpl <em>Style Map</em>}' class.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @see org.eclipse.birt.chart.model.attribute.impl.StyleMapImpl
	 * @see org.eclipse.birt.chart.model.attribute.impl.AttributePackageImpl#getStyleMap()
	 * @generated
	 */
	int STYLE_MAP = 31;
	/**