minor refactoring

lib/single-file/single-file-helper.js

@@ -157,12 +157,13 @@ this.singlefile.lib.helper = this.singlefile.lib.helper || (() => {
 			}
 		}
 		if (element.tagName == "IMG") {
-			element.setAttribute(IMAGE_ATTRIBUTE_NAME, data.images.length);
 			const imageData = {
 				currentSrc: elementHidden ?
 					"data:image/gif;base64,R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==" :
 					(options.loadDeferredImages && element.getAttribute(LAZY_SRC_ATTRIBUTE_NAME)) || element.currentSrc
 			};
+			data.images.push(imageData);
+			element.setAttribute(IMAGE_ATTRIBUTE_NAME, data.images.length - 1);
 			element.removeAttribute(LAZY_SRC_ATTRIBUTE_NAME);
 			computedStyle = computedStyle || win.getComputedStyle(element);
 			if (computedStyle) {
@@ -179,7 +180,6 @@ this.singlefile.lib.helper = this.singlefile.lib.helper || (() => {
 					imageData.objectPosition = computedStyle.getPropertyValue("object-position");
 				}
 			}
-			data.images.push(imageData);
 		}
 		if (element.tagName == "VIDEO") {
 			if (!element.poster) {