Use JPEG for background image on intro page to improve loading speed

The text is still pretty sharp so using PNG seems very needles and JPEG is
much smaller (eventhough the PNG has already been optimized via optipng).

css/specifics.css

@@ -35,7 +35,7 @@
 .banner {
     margin: -23px;
     margin-bottom: 10px;
-    background-image: url(../img/screenshots/plasma.png);
+    background-image: url(../img/screenshots/plasma.jpg);
     background-position-y: -758px;
     background-position-x: right;
     background-repeat: no-repeat;