Have you tried creating a separate part for the image, and then attach it to the rest of the message? Using Thunderbird, image1 is shown whithin the message, and image2 as an attachment:

...

$msg->attach(
 Type =>'image/gif',
 Path =>'/tmp/image1.gif',
 Disposition => 'attachment'
);

my $part = MIME::Lite->new(
 Path => '/foo/image2.gif'
);

$part->attr('Content-Type' => 'image/gif');
$msg->attach($part);

...
[download]