fix the jumping beans effect

Pages will jump if image size isn't explicitly declared.

contrib/js/nntpchan/banner.js

@@ -7,6 +7,8 @@ function nntpchan_inject_banners(elem, prefix) {
   var e = document.createElement("img");
   e.src = banner;
   e.id = "nntpchan_banner";
+  e.height = "150";
+  e.width = "300";
   elem.appendChild(e);
 }