Add alternative extraction method.

Looks at the srcset of the image on the page, which has the advantage of getting
the currently selected pictures if there are multiple in the set.

extension/manifest.json

@@ -21,5 +21,5 @@
         "activeTab",
         "downloads"
     ],
-    "version": "1.0.1"
+    "version": "1.0.2"
 }

extension/script.js

@@ -1,36 +1,61 @@
-var imageUrl = document.querySelector("meta[property='og:image']").getAttribute("content");
-var filename;
-
-try {
-    // We can't access the window._sharedData variable directly, since we're in
-    // an isolated world. But we can extract the script text and parse it.
-    var sharedDataScriptNode = Array.prototype.find.call(
-        document.querySelectorAll("script"),
-        node => node.textContent.includes("window._sharedData"));
-    var sharedDataJson = sharedDataScriptNode.textContent;
-    sharedDataJson = sharedDataJson
-        .replace(/^window\._sharedData\s*=\s*/, "")
-        .replace(/;*\s*$/, "");
-    var sharedData = JSON.parse(sharedDataJson);
-
-    var caption = sharedData.entry_data.PostPage[0].media.caption;
+function run() {
+    try {
+        const imageNode = document.querySelector("img[srcset]");
+        const srcMap = new Map(imageNode.srcset
+            .split(",")
+            .map(src => src.split(" ").reverse()));
+        const imageUrl = srcMap.get("1080w");
+
+        chrome.runtime.sendMessage({
+            type: "download",
+            url: imageUrl,
+            filename: generateFilename(imageNode.alt)
+        });
+        return;
+    } catch (err) {
+        console.warn("Could not extract current image, falling back on metadata.");
+    }
+
+    var imageUrl = document.querySelector("meta[property='og:image']")
+        .getAttribute("content");
+    var filename;
+
+    try {
+        // We can't access the window._sharedData variable directly, since we're in
+        // an isolated world. But we can extract the script text and parse it.
+        var sharedDataScriptNode = Array.prototype.find.call(
+            document.querySelectorAll("script"),
+            node => node.textContent.includes("window._sharedData"));
+        var sharedDataJson = sharedDataScriptNode.textContent;
+        sharedDataJson = sharedDataJson
+            .replace(/^window\._sharedData\s*=\s*/, "")
+            .replace(/;*\s*$/, "");
+        var sharedData = JSON.parse(sharedDataJson);
+
+        var caption = sharedData.entry_data.PostPage[0].media.caption;
+        filename = generateFilename(caption);
+    } catch (ex) {};
+
+    chrome.runtime.sendMessage({
+        type: "download",
+        url: imageUrl,
+        filename: filename
+    });
+}
+
+function generateFilename(caption) {
     // Remove hashtags, question marks and trailing periods.
     caption = caption
         .replace(/#\w+/g, "")
         .trim()
-        .replace(/[?:]/g, "")
+        .replace(/[?:"]/g, "")
         .replace(/\.+$/, "")
         .trim();
 
     if (caption.length > 200) {
         caption = caption.substring(0, 200);
     }
+    return caption + ".jpeg";
+}
 
-    filename = caption + ".jpeg";
-} catch (ex) {};
-
-chrome.runtime.sendMessage({
-    type: "download",
-    url: imageUrl,
-    filename: filename
-});
+run();