Ubuntu Networking :: Apache Error But Only Via Network

I have an Apache set up to server some static html and php files which all work fine. The php and statics include image links which fail, but only when accessed over a real network - if I connect to http://localhost or http://[myIP address] from the system itself, I get images just fine. The failure in Firefox is "The image http://blah-blah.png cannot be displayed because it contains errors" (any image, have used binary diff to ensure no copying errors)

Some details:
The system is running Ubuntu 10.4, with all the latest patches, and apache 2.2.14-5ubuntu8
Doing wget of a php/html file results in precisely the correct content being saved. Doing the same with the PNG I get the file, but it has approx 294 bytes of garbage before the <89>PNG, which includes the http header:

(binary stuff cut)HTTP/1.1 200 OK^M
Date: Tue, 04 May 2010 13:28:41 GMT^M
Server: Apache/2.2.14 (Ubuntu)^M
Last-Modified: Tue, 04 May 2010 08:29:36 GMT^M
ETag: "9fb4f-1004-485c082855fd1"^M
Accept-Ranges: bytes^M
Content-Length: 4100^M
Keep-Alive: timeout=15, max=100^M
Connection: Keep-Alive^M
Content-Type: image/png^M
^M
<89>PNG^M...

I have an Atheros Communications AR8131 Gigabit Ethernet controller using DHCP for network config. Everything else seems to be working exactly as expected. I re-iterate, if I connect from the system to it's own IP address. I have tried connecting from various clients including non-Linux ones, images get served with garbage up front I have no idea where to start debugging this (having worked out what is really happening).