Add image constructor

Server/MarkdownPreviewer/pandoc.py

@@ -125,3 +125,19 @@ class PString:
                 't' : 'Str',
                 'c' : self.content
                 }
+
+class Image:
+    def __init__(self, attr, caption, url):
+        self.attr = attr
+        self.caption = caption
+        self.url = url
+
+    def toJson(self):
+        return {
+                't' : 'Image',
+                'c' : [
+                    self.attr.toJson(),
+                    self.caption,
+                    [self.url, ""]
+                ]
+                }