Image puzzle

http://www.lettice.net/yarn1.jpg

HTTP/1.1 200 OK
Date: Thu, 03 Feb 2005 16:42:28 GMT
Server: tigershark/3.0.114 (dn6.directnic.com)
Last-Modified: Tue, 01 Feb 2005 11:25:40 GMT
Connection: close
Content-Type: image/jpeg
Content-Length: 12964

http://www.stevepugh.net/lettice_pics/yarn1.jpg

HTTP/1.1 200 OK
Date: Thu, 03 Feb 2005 16:42:22 GMT
Server: Apache/1.3.33 (Unix) mod_gzip/1.3.26.1a mod_auth_passthrough/1.8 mod_log_bytes/1.2 mod_bwlimited/1.4 PHP/4.3.9 FrontPage/5.0.2.2635 mod_ssl/2.8.22 OpenSSL/0.9.7a
Last-Modified: Wed, 02 Feb 2005 10:34:02 GMT
ETag: "a04b5-32a4-4200ac9a"
Accept-Ranges: bytes
Content-Length: 12964
Connection: close
Content-Type: image/jpeg

Why does the first one not display for a large number of people but the second one displays with no problems? The image files themselves are identical.