# in the parent CGI script
my $id = get_user_id;          # just give them a new ID
build_image($id, @arguments);  # or store the arguments
                               # in a database or something

print "<img src='/cgi-bin/fetch_image?$id'>\n";

# or you can be cool and use CGI PATH_INFO stuff:

print "<img src='/cgi-bin/fetch_image.cgi/$id.gif'>";