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/util/AttributeValidator.java
context );
			case AttributePackage.URL_VALUE :
				return validateURLValue( (URLValue) value, diagnostics, context );
			case AttributePackage.ACTION_TYPE :
				return validateActionType( (Object) value, diagnostics, context );
			case AttributePackage.ANCHOR :
				return validateAnchor( (Object) value, diagnostics, context );
			case AttributePackage.ANGLE_TYPE :
				return validateAngleType( (Object) value, diagnostics, context );
			case AttributePackage.AXIS_TYPE :
				return validateAxisType( (Object) value, diagnostics, context );
			case AttributePackage.CHART_DIMENSION :
				return validateChartDimension( (Object) value,
						diagnostics,
						context );
			case AttributePackage.CHART_TYPE :
				return validateChartType( (Object) value, diagnostics, context );
			case AttributePackage.DATA_POINT_COMPONENT_TYPE :
				return validateDataPointComponentType( (Object) value,
						diagnostics,
						context );
			case AttributePackage.DATA_TYPE :
				return validateDataType( (Object) value, diagnostics, context );
			case AttributePackage.DATE_FORMAT_DETAIL :
				return validateDateFormatDetail( (Object) value,
						diagnostics,
						context );
			case AttributePackage.DATE_FORMAT_TYPE :
				return validateDateFormatType( (Object) value,
						diagnostics,