The idea behind this is that you typically already have
images on your web page; so you replace the SRC of one
of the images with a URL that routes through this logger,
but still returns the image. So the browser doesn't know
the difference, but the server does.
So what you should actually do is just use one of the images
you already have on your site. You say that the image you
want to use is at /home/username/www/cgi-bin/showpic/spacer.gif? That's not
right. "showpic" is the name of the CGI script, so it
can't also be a directory on the filesystem.
Let's say that you currently have an image like this on
your web page:
And the actual filesystem location of this image is:
You would replace the IMG tag with this:
And, in showpic (the CGI script), you'd set the $home
my $home = "/home/bar/www";