You're reading your image into a variable and passing it to dompdf. As such dompdf is treating that text that makes up that image as HTML source. Since your image is readable from the PHP script at the location stored in $LgPath
you should be able to do something like this:
<?php
require_once("dompdf/dompdf_config.inc.php");
$LgPath=$_POST['image'];
$html = '<img src="' . $LgPath . '">';
$dompdf = new DOMPDF();
$dompdf->load_html($html);
$dompdf->render();
$dompdf->stream("sample.pdf");
?>