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
return validateInsets( (Insets) value, diagnostics, context );
			case AttributePackage.INTERACTIVITY :
				return validateInteractivity( (Interactivity) value,
						diagnostics,
						context );
			case AttributePackage.JAVA_DATE_FORMAT_SPECIFIER :
				return validateJavaDateFormatSpecifier( (JavaDateFormatSpecifier) value,
						diagnostics,
						context );
			case AttributePackage.JAVA_NUMBER_FORMAT_SPECIFIER :
				return validateJavaNumberFormatSpecifier( (JavaNumberFormatSpecifier) value,
						diagnostics,
						context );
			case AttributePackage.LINE_ATTRIBUTES :
				return validateLineAttributes( (LineAttributes) value,
						diagnostics,
						context );
			case AttributePackage.LOCATION :
				return validateLocation( (Location) value, diagnostics, context );
			case AttributePackage.LOCATION3_D :
				return validateLocation3D( (Location3D) value,
						diagnostics,
						context );
			case AttributePackage.MARKER :
				return validateMarker( (Marker) value, diagnostics, context );
			case AttributePackage.NUMBER_FORMAT_SPECIFIER :
				return validateNumberFormatSpecifier( (NumberFormatSpecifier) value,
						diagnostics,
						context );
			case AttributePackage.PALETTE :