I changed the wiki.png to a logo of my own and everything was working fine. I was using Netscape 7.1 and tested it and all looked ok.
However after a few hours I closed my browser and restarted it only to find that png no longer showed.
I tried in IE and the png displayed correctly ...
Is this a Netscape 7.1 related problem?
In both cases I tried as a regular 'viewer' and after I had logged in but no difference.