If you want to serve PNG images, why use the browser string?
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Accept-Encoding: gzip,deflate
Accept-Language: en-us,en;q=0.5
Connection: keep-alive
Host: pgl.yoyo.org
Keep-Alive: 300
Referer: http://www.google.com/search?hs=zky&hl=en&lr=&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&q=browser+headers&btnG=Search
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8) Gecko/20051111 Firefox/1.5
String match image/png in the Accept: header!
(headers courtesy of http://pgl.yoyo.org/http/browser-headers.php)
