Because the URL is returning a PNG. Here's the output of HttpFox in Firefox:
Note the Type column.
Also:
$ curl -D headers 'http://geobeck.tripod.com/bin/counter/~geobeck/jokehold/jokes.html' >jokes $ cat headers HTTP/1.1 200 OK Date: Sat, 31 Oct 2015 16:50:25 GMT Server: Squeegit/1.2.5 (3_sir) Set-Cookie: CookieStatus=COOKIE_OK; path=/; domain=.tripod.com; expires=Sun, 30-Oct-2016 16:50:25 GMT Vary: * X-Server-IP: 209.202.244.196 P3P: policyref="http://www.lycos.com/w3c/p3p.xml", CP="IDC DSP COR CURa ADMa DEVa CUSa PSAa IVAa CONo OUR IND UNI STA" Content-Length: 150 Expires: Thu, 01 Jan 1970 00:16:40 GMT Connection: close Content-Type: image/png Note the Content-Type header.
$ file jokes jokes: PNG image, 70 x 11, 1-bit colormap, non-interlaced PNGs don't have "source", at least in the way a browser allows Viewing Source.