PHP Image Host - Sending HTTP Headers Twice?

I have a php script which serves an image. It's very simple: header('Content-Type: image/' . $ImageData['content_type']); readfile($File); When viewing the script with the Firefox Extension: LiveHTTPHeaders, it gives the following output for a SINGLE request: ---------------------------------------------------------- https://domain.tld/img.php?i=260 GET /img.php?i=260 HTTP/1.1 Host: domain.tld User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: UTF-8,* Keep-Alive: 115 Connection: keep-alive Cookie: session=blahblah Cache-Control: max-age=0 HTTP/1.1 200 OK Server: nginx Date: Mon, 10 May 2010 20:17:09 GMT Content-Type: image/jpeg Transfer-Encoding: chunked Connection: keep-alive ---------------------------------------------------------- https://domain.tld/img.php?i=260 GET /img.php?i=260 HTTP/1.1 Host: domain.tld User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8 Accept-Language: en-us,en;q=0.5 Accept-Encoding: gzip,deflate Accept-Charset: UTF-8,* Keep-Alive: 115 Connection: keep-alive HTTP/1.1 200 OK Server: nginx Date: Mon, 10 May 2010 20:17:10 GMT Content-Type: text/html; charset=utf-8 Transfer-Encoding: chunked Connection: keep-alive Content-Encoding: gzip ---------------------------------------------------------- As you can see, the browser is requesting the image twice, and PHP is sending two different Content-Type headers. Why is this?