excerpt with image is the image is the first thing in the post

gistfile1.txt

/**
 * [the_custom_excerpt description]
 * @param  [type] $post_content [description]
 * @return [type]               [description]
 */
function get_the_custom_excerpt( $post_content ) {

	$caption_img = '';
	$excerpt = '';

	// checking for a caption
	preg_match( "#(\[caption)([\s\S]+)(\[/caption\])#", $post_content, $caption_match );
	$caption_pos = strpos( $post_content, '[caption' );

	if ( 0 == $caption_pos && false != $caption_match ) {
		// we have an image with a caption at the start of the post
		$para1 = explode( $caption_match[0], $post_content ); // isolating the caption shortcode
		$para2 = explode( "\n", $para1[1] ); // exploding the content again

		// removing empty lines
		foreach ( $para2 as $k => $v ) {
			if ( 0 == strlen( $v ) || 1 == strlen( $v ) ) {
				unset( $para2[ $k ] );
			}
		}
		// resetting array keys
		$para2 = array_values( $para2 );
		return $caption_match[0] . "\n" . $para2[0];
	} else {
		// we have an image without a caption at the start of the post
		// breaking into array
		$para = explode( "\n", $post_content );

		// removing empty lines
		foreach( $para as $k => $v ) {
			if ( 1 == strlen( $v ) ) {
				unset(  $para[ $k ] );
			}
		}

		foreach( $para as $k => $v ) {
			if ( 0 == $k && false != strpos( $v, '<img' ) ) {
				return $para[0] . "\n" . $para[1];
			} else {
				return $para[0];
			}
		}
	}
}