It looks like the image is a graph that changes all the time, so using an inline base64 encoded image might make sense here. Something like:
#!/usr/bin/perl
use strict;
use warnings;
use MIME::Base64 qw(encode_base64url);
my $img="/var/www/grafika/graphHour.png";
open(IMG,$img) or die "Can't open $img: $!\n";
my $imgdata;
{local $/=undef;$imgdata=<IMG>;}
close IMG;
my $b64=encode_base64url($imgdata);
print "Content-type: text/html\n\n ";
print "<html> \n";
print "<head></head>\n";
print "<body>\n";
print "<img src='data:image/png;base64,$b64'>";
print "</body></html>"
