gistfile1.php

<?php

$src = '';

$content = get_the_content();

/* Content contains only a url. */
$src = esc_url( trim( $content ) );

if ( ! empty( $src ) && is_array( @getimagesize( $src ) ) ) {
	var_dump( $src );
}

/* Attempt to match the first image tag. */
preg_match( '/<img (.+?)>/', $content, $matches );

/* First image was found, extract source and sanitize. */
$atts = array();
if ( isset ( $matches[1] ) ) {
	foreach ( wp_kses_hair( $matches[1], array( 'http' ) ) as $attr => $value ) {
		$atts[$attr] = $value['value'];
	}
	if ( isset( $atts['src'] ) && ! empty( $atts['src'] ) ) {
		$src = esc_url( $atts['src'] );
	}
	if ( ! is_array( @getimagesize( $src ) ) ) {
		$src = '';
	}
}

print '<pre>'; print_r( $src ); print '</pre>';
print '<pre>'; print_r( $atts ); print '</pre>';
?>