Try it this way:

use LWP;

my $ua = LWP::UserAgent->new;
my $req = HTTP::Request->new( POST => $url);

$req->content_type('image/jpeg');
open(FILE,$filename);
$req->content(join('',<FILE>));
close(FILE);

my $res = $ua->request($req);

die("Bzzzzzzz, error!") unless $res->is_success;
[download]

You must set proper content type. And I just can't figure out why are you trying to base64 encode the file. Without the headers is base64 encoding pointless.

If server does not support direct upload of picture this way, you should probably encode it as a part of HTML form. Use content type of 'multipart/form-data' and you should probably use some (MIME?) modules to build up a content with proper headers.