I'm trying to send an image along with notification via php!

I'm trying to send an image next to the notification via php, the notification is sent normally, but the image will not. Can you tell me what's wrong with this code?

<?php


if (isset($_POST['titulo'])) {

    move_uploaded_file($_FILES['Imagem']['tmp_name'], 'Imagens/' . $_FILES['Imagem']['name']);


    function sendMessage()
    {
        $content = array(
            "en" => $_POST['mensagem']
        );

        $imagem = $_FILES['Imagem'];
        $headings = array("en" => $_POST['titulo']);
        $fields = array(
            'app_id' => "2ac0abda-15ae-4d27-aa5b-da8400941d97",
            'included_segments' => array('All'),
            'contents' => $content,
            'headings' => $headings,
            //'small_icon'
            'large_icon' => 'https://www.iconexperience.com/_img/v_collection_png/64x64/shadow/ok.png', //  Ok
            'big_picture' => $imagem, //  Ok
            // 'android_led_color' => "FF0000FF",
            //'notification[android_accent_color]' => "FF0000FF",

        );


        $fields = json_encode($fields);

        $ch = curl_init();
        curl_setopt($ch, CURLOPT_URL, "https://onesignal.com/api/v1/notifications");
        curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json',
            'Authorization: Basic M2I5MDEzYTItN2VkYS00YTVhLWE0OWMtMzY3MzNkOTU3NWQ3'));
        curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
        curl_setopt($ch, CURLOPT_HEADER, FALSE);
        curl_setopt($ch, CURLOPT_POST, TRUE);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);

        $response = curl_exec($ch);
        curl_close($ch);

        return $response;

    }

    $response = sendMessage();
    $return["allresponses"] = $response;
    $return = json_encode($return);

}