You can

1. use the Alias directive to have a location point to some path outside the DocumentRoot
2. allow the webserver process to write to some images directory inside the tree rooted at DocumentRoot
3. use a CGI script to serve the image. Provide e.g. the tag

   <img src="/cgi-bin/image.pl?img=$tag.jpg">
   [download]

   in the page referencing the image, and have it served thusly:

   #!/usr/bin/perl -T
   use CGI;
   use strict;
   my $q = new CGI;
   if($q->param('img') =~ /^([\w\.\-]+)$/) { # untaint
       my $file = $1;
       if(-f "/tmp/$file") {
           if(open(I,'<',"/tmp/$file")) {
               print $q->header(-content_type => "image/jpeg");
               print while <I>;
               close I;
               exit; 
           }   
       }   
   }   
   print $q->header(-status => '404 Not found');
   print "<h1>Not found.</h1>\n";
   [download]

--shmem

_($_=" "x(1<<5)."?\n".q·/)Oo.  G°\        /
                              /\_¯/(q    /
----------------------------  \__(m.====·.(_("always off the crowd"))."·
");sub _{s./.($e="'Itrs `mnsgdq Gdbj O`qkdq")=~y/"-y/#-z/;$e.e && print}