+def add_image_sizes(tree, gallery_path, gallery_url, base_url):
+    widths = [360, 600, 1200, 1800, 2400]
+    try:
+        os.makedirs(gallery_path)
+    except:
+        pass
+
+    for i, ilustr in enumerate(tree.findall('//ilustr')):
+        rel_path = ilustr.attrib['src']
+        img_url = urllib.parse.urljoin(base_url, rel_path)
+
+        f = urllib.request.urlopen(img_url)
+        img = Image.open(f)
+        ext = {'GIF': 'gif', 'PNG': 'png'}.get(img.format, 'jpg')
+
+        srcset = []
+        # Needed widths: predefined and original, limited by
+        # whichever is smaller.
+        img_widths = [
+            w for w in
+            sorted(
+                set(widths + [img.size[0]])
+            )
+            if w <= min(widths[-1], img.size[0])
+        ]
+        largest = None
+        for w in widths:
+            fname = '%d.W%d.%s' % (i, w, ext)
+            fpath = gallery_path + fname
+            if not os.path.exists(fpath):
+                height = round(img.size[1] * w / img.size[0])
+                th = img.resize((w, height))
+                th.save(fpath)
+            th_url = gallery_url + fname
+            srcset.append(" ".join((
+                th_url,
+                '%dw' % w
+            )))
+            largest_url = th_url
+        ilustr.attrib['srcset'] = ", ".join(srcset)
+        ilustr.attrib['src'] = largest_url
+
+        f.close()
+
+
+def transform(wldoc, stylesheet='legacy', options=None, flags=None, css=None, gallery_path='img/', gallery_url='img/', base_url='file://./'):