The error(s) I get in the apache2 error log are:-
[Thu Nov 24 13:41:50 2005] [error] [client 193.128.168.194] (13)Permission denied: exec of '/var/www/cgi-bin/twiki/pub/TWiki/PlainSkin/fullscreen.gif' failed, referer: http://my.system.net/cgi-bin/twiki/bin/view/TWiki/TWikiSkinBrowser?skin=plai... [Thu Nov 24 13:41:50 2005] [error] [client 193.128.168.194] Premature end of script headers: fullscreen.gif, referer: http://my.system.net/cgi-bin/twiki/bin/view/TWiki/TWikiSkinBrowser?skin=plai...
It appears that the perl in the TWiki is trying to execute the fullscreen.gif file. Previously I had execute permission set on the file and then I was getting "(8)Exec format error:" but otherwise the result was the same.
I don't think it's an apache error as GIFs elsewhere seem to be handled OK, it's just within the TWiki that its screwy.