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/impl/AttributeFactoryImpl.java
if ( result == null )
					throw new IllegalArgumentException( "The value '" + initialValue + "' is not a valid enumerator of '" + eDataType.getName( ) + "'" ); 
				return result;
			}
			case AttributePackage.UNITS_OF_MEASUREMENT :
			{
				UnitsOfMeasurement result = UnitsOfMeasurement.get( initialValue );
				if ( result == null )
					throw new IllegalArgumentException( "The value '" + initialValue + "' is not a valid enumerator of '" + eDataType.getName( ) + "'" ); 
				return result;
			}
			case AttributePackage.VERTICAL_ALIGNMENT :
			{
				VerticalAlignment result = VerticalAlignment.get( initialValue );
				if ( result == null )
					throw new IllegalArgumentException( "The value '" + initialValue + "' is not a valid enumerator of '" + eDataType.getName( ) + "'" ); 
				return result;
			}
			case AttributePackage.ACTION_TYPE_OBJECT :
				return createActionTypeObjectFromString( eDataType,
						initialValue );
			case AttributePackage.ANCHOR_OBJECT :
				return createAnchorObjectFromString( eDataType, initialValue );
			case AttributePackage.ANGLE_TYPE_OBJECT :
				return createAngleTypeObjectFromString( eDataType, initialValue );
			case AttributePackage.AXIS_TYPE_OBJECT :
				return createAxisTypeObjectFromString( eDataType, initialValue );
			case AttributePackage.CHART_DIMENSION_OBJECT :
				return createChartDimensionObjectFromString( eDataType,
						initialValue );