Patch by Tasos Sahanidis
Ubuntu 15.04 doesn't return the icons in 16x16 so the FORCE_SIZE flag needs to be used.
Additionally, external icons need to be scaled to match the rest.
--- a/src/mainwindow.cc
+++ b/src/mainwindow.cc
@@ -241,14 +241,19 @@
     theme = Gtk::IconTheme::get_default();
 
     try {
-        pixbuf = theme->load_icon(name, width, Gtk::ICON_LOOKUP_GENERIC_FALLBACK);
+        pixbuf = theme->load_icon(name, width, Gtk::ICON_LOOKUP_GENERIC_FALLBACK | Gtk::ICON_LOOKUP_FORCE_SIZE);
 
         if (pixbuf)
             i->set(pixbuf);
-        else
-            i->set(name);
+        else {
+            Glib::RefPtr<Gdk::Pixbuf> pixbuf = Gdk::Pixbuf::create_from_file(name);
+            pixbuf = pixbuf->scale_simple(width, height, Gdk::INTERP_BILINEAR);
+            i->set(pixbuf);
+	}
     } catch (Gtk::IconThemeError &e) {
-        i->set(name);
+        Glib::RefPtr<Gdk::Pixbuf> pixbuf = Gdk::Pixbuf::create_from_file(name);
+        pixbuf = pixbuf->scale_simple(width, height, Gdk::INTERP_BILINEAR);
+        i->set(pixbuf);
     } catch (Gio::Error &e) {
         i->set(name);
     }