avoid overriding existing background images

lib/single-file/html-images-minifier.js

@@ -68,7 +68,7 @@ this.imagesMinifier = this.imagesMinifier || (() => {
 
 	function processImages(doc, duplicates, options, customProperties) {
 		doc.querySelectorAll("img[src]:not([srcset])").forEach((imgElement, imgIndex) => {
-			if (duplicates.has(imgElement)) {
+			if (duplicates.has(imgElement) && imgElement.style.getPropertyValue("background-image")) {
 				const src = imgElement.getAttribute("src");
 				const dataAttributeName = docHelper.imagesAttributeName(options.sessionId);
 				const imageData = options.imageData[Number(imgElement.getAttribute(dataAttributeName))];