You can use the example below to add a static image header to all emails sent from your forms. Replace the URL below with an image URL of your choice.

add_filter('frm_email_message', 'add_email_header', 10, 2);
function add_email_header($message, $atts) {
 //edit the email header image source to include an image URL of your choice
 $email_header = '<img src="http://www.yoursite.com/images/email-header.jpg" alt="Header" /><br />';
 $message = $email_header . $message;
 return $message;
}

For a clear and concise summary displayed next to your subject line, use this code snippet to add a preheader to your emails.

add_filter('frm_email_message', 'add_email_header', 10, 2);
 function add_email_header($message, $atts) {
     if (isset($atts['form']) && $atts['form']->id == 1) { //replace 1 with the ID of the form
         $email_header = 'This is the preheader text';
         $message = $email_header . $message;
     }
     return $message;
 }