I'm not familiar with Mac tools. All I can say is that the reason for the image not showing as expected almost certainly is the fact that the output starts prematurely.
This is the http traffic sniffed with live HTTP headershttp://www.brunopaccard.com/cpg/captcha.php?lang=french
GET /cpg/captcha.php?lang=french HTTP/1.1
Host: www.brunopaccard.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 ( .NET CLR 3.5.30729)
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Referer: http://forum.coppermine-gallery.net/index.php?action=post;topic=64916.0;num_replies=4
Cookie: faf22696d326577ad2dbc65575e7f336=d93f35be736ad61724b9accccbfc20f6; PHPSESSID=972f0e5d378b5713385bf3aac18ff220; coppermine_brunopaccard_data=YTozOntzOjI6IklEIjtzOjMyOiI1OWNiZjM5MTM4N2Q4ZDg4ZTkwNDRhNzIyNDc0M2NiNyI7czoyOiJhbSI7aToxO3M6NDoibGFuZyI7czo2OiJmcmVuY2giO30%3D
Cache-Control: max-age=0
HTTP/1.1 200 OK
Date: Wed, 12 May 2010 15:59:24 GMT
Server: Apache
Cache-Control: no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Pragma: no-cache
Keep-Alive: timeout=15, max=99
Connection: Keep-Alive
Transfer-Encoding: chunked
Content-Type: image/jpeg
----------------------------------------------------------
http://nolobe.com/mint/pepper/tillkruess/durations/recalc.php?token=756897106&time=90
GET /mint/pepper/tillkruess/durations/recalc.php?token=756897106&time=90 HTTP/1.1
Host: nolobe.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 ( .NET CLR 3.5.30729)
Accept: image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Referer: http://nolobe.com/interarchy/
Cookie: MintAcceptsCookies=1; MintUnique=1; MintUniqueHour=1273676400; MintUniqueDay=1273669200; MintUniqueWeek=1273323600; MintUniqueMonth=1272632400; MintUniqueLocation=1; MintDurationsToken=756897106; __utma=1.456312270.1273679909.1273679909.1273679909.1; __utmb=1.1.10.1273679909; __utmc=1; __utmz=1.1273679909.1.1.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=Interarchy
HTTP/1.1 200 OK
Date: Wed, 12 May 2010 15:59:32 GMT
Server: Apache/2.2.9 (Debian) PHP/5.2.6-1+lenny8 with Suhosin-Patch
X-Powered-By: PHP/5.2.6-1+lenny8
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 20
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
----------------------------------------------------------
http://nolobe.com/mint/pepper/tillkruess/durations/recalc.php?token=756897106&time=105
GET /mint/pepper/tillkruess/durations/recalc.php?token=756897106&time=105 HTTP/1.1
Host: nolobe.com
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; de; rv:1.9.2.3) Gecko/20100401 Firefox/3.6.3 ( .NET CLR 3.5.30729)
Accept: image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language: de-de,de;q=0.8,en-us;q=0.5,en;q=0.3
Accept-Encoding: gzip,deflate
Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive: 115
Connection: keep-alive
Referer: http://nolobe.com/interarchy/
Cookie: MintAcceptsCookies=1; MintUnique=1; MintUniqueHour=1273676400; MintUniqueDay=1273669200; MintUniqueWeek=1273323600; MintUniqueMonth=1272632400; MintUniqueLocation=1; MintDurationsToken=756897106; __utma=1.456312270.1273679909.1273679909.1273679909.1; __utmb=1.1.10.1273679909; __utmc=1; __utmz=1.1273679909.1.1.utmcsr=google|utmccn=(organic)|utmcmd=organic|utmctr=Interarchy
HTTP/1.1 200 OK
Date: Wed, 12 May 2010 15:59:47 GMT
Server: Apache/2.2.9 (Debian) PHP/5.2.6-1+lenny8 with Suhosin-Patch
X-Powered-By: PHP/5.2.6-1+lenny8
Vary: Accept-Encoding
Content-Encoding: gzip
Content-Length: 20
Keep-Alive: timeout=15, max=99
Connection: Keep-Alive
Content-Type: text/html
----------------------------------------------------------