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
* Returns the meta object for data type '{@link org.eclipse.birt.chart.model.attribute.Position <em>Position Object</em>}'.
	 * <!-- begin-user-doc --> <!-- end-user-doc -->
	 * @return the meta object for data type '<em>Position Object</em>'.
	 * @see org.eclipse.birt.chart.model.attribute.Position
	 * @model instanceClass="org.eclipse.birt.chart.model.attribute.Position"
	 *        extendedMetaData="name='Position:Object' baseType='Position'" 
	 * @generated
	 */
	EDataType getPositionObject( );
	/**
	 * Returns the meta object for data type '<em>RGB Value</em>'. <!--
	 * begin-user-doc --> <!-- end-user-doc -->
	 * 
	 * @return the meta object for data type '<em>RGB Value</em>'.
	 * @model instanceClass="int"
	 * @generated
	 */
	EDataType getRGBValue( );
	/**
	 * Returns the meta object for data type '
	 * {@link java.lang.Integer <em>RGB Value Object</em>}'. <!--
	 * begin-user-doc --> <!-- end-user-doc -->
	 * 
	 * @return the meta object for data type '<em>RGB Value Object</em>'.
	 * @see java.lang.Integer
	 * @model instanceClass="java.lang.Integer"
	 * @generated
	 */
	EDataType getRGBValueObject( );
	/**