A: After trying out all possible (and obviously impossible) settings I got it working using: /wiki/db/wiki.gif in the config file.
A2: The above suggestion doesn't work for me. I did take a look at the source code of wiki.perl, the related lines are:
$logoImage = "img src=\"$LogoUrl\" alt=\"$altText\" border=0"; $header = &ScriptLink($HomePage, "<$logoImage>");
namely $logoImage should hold the relative path of wiki.gif from your wiki.perl. In your case, I suppose your wiki.perl is located at /public_html/wiki/bin/wiki.perl, while your wiki.gif is located at /public_html/wiki/db/wiki.gif, then $logoImage should be set to "../wiki/db/wiki.gif"... and it works for mine.