Add image file upload field to WooCommerce single products

You could try the following, that will store uploaded image data as custom cart item data and save it as custom order item meta data: 

// Display additional product fields (+ jQuery code)

add_action( 'woocommerce_before_add_to_cart_button', 'display_additional_product_fields', 9 );
function display_additional_product_fields(){
    ?>
    <p class="form-row validate-required" id="image" >
        <label for="file_field"><?php echo __("Upload Image") . ': '; ?>
            <input type='file' name='image' accept='image/*'>
        </label>
    </p>
    <?php
}


// Add custom fields data as the cart item custom data
add_filter( 'woocommerce_add_cart_item_data', 'add_custom_fields_data_as_custom_cart_item_data', 10, 2 );
function add_custom_fields_data_as_custom_cart_item_data( $cart_item, $product_id ){
    if( isset($_FILES['image']) && ! empty($_FILES['image']) ) {
        $upload       = wp_upload_bits( $_FILES['image']['name'], null, file_get_contents( $_FILES['image']['tmp_name'] ) );
        $filetype     = wp_check_filetype( basename( $upload['file'] ), null );
        $upload_dir   = wp_upload_dir();
        $upl_base_url = is_ssl() ? str_replace('http://', 'https://', $upload_dir['baseurl']) : $upload_dir['baseurl'];
        $base_name    = basename( $upload['file'] );

        $cart_item['file_upload'] = array(
            'guid'      => $upl_base_url .'/'. _wp_relative_upload_path( $upload['file'] ), // Url
            'file_type' => $filetype['type'], // File type
            'file_name' => $base_name, // File name
            'title'     => ucfirst( preg_replace('/\.[^.]+$/', '', $base_name ) ), // Title
        );
        $cart_item['unique_key'] = md5( microtime().rand() ); // Avoid merging items
    }
    return $cart_item;
}

// Display custom cart item data in cart (optional)
add_filter( 'woocommerce_get_item_data', 'display_custom_item_data', 10, 2 );
function display_custom_item_data( $cart_item_data, $cart_item ) {
    if ( isset( $cart_item['file_upload']['title'] ) ){
        $cart_item_data[] = array(
            'name' => __( 'Image uploaded', 'woocommerce' ),
            'value' =>  str_pad($cart_item['file_upload']['title'], 16, 'X', STR_PAD_LEFT) . '…',
        );
    }
    return $cart_item_data;
}

// Save Image data as order item meta data
add_action( 'woocommerce_checkout_create_order_line_item', 'custom_field_update_order_item_meta', 20, 4 );
function custom_field_update_order_item_meta( $item, $cart_item_key, $values, $order ) {
    if ( isset( $values['file_upload'] ) ){
        $item->update_meta_data( '_img_file',  $values['file_upload'] );
    }
}

// Admin orders: Display a linked button + the link of the image file
add_action( 'woocommerce_after_order_itemmeta', 'backend_image_link_after_order_itemmeta', 10, 3 );
function backend_image_link_after_order_itemmeta( $item_id, $item, $product ) {
    // Only in backend for order line items (avoiding errors)
    if( is_admin() && $item->is_type('line_item') && $file_data = $item->get_meta( '_img_file' ) ){
        echo '<p><a href="'.$file_data['guid'].'" target="_blank" class="button">'.__("Open Image") . '</a></p>'; // Optional
        echo '<p><code>'.$file_data['guid'].'</code></p>'; // Optional
    }
}

// Admin new order email: Display a linked button + the link of the image file
add_action( 'woocommerce_email_after_order_table', 'wc_email_new_order_custom_meta_data', 10, 4);
function wc_email_new_order_custom_meta_data( $order, $sent_to_admin, $plain_text, $email ){
    // On "new order" email notifications
    if ( 'new_order' === $email->id ) {
        foreach ($order->get_items() as $item ) {
            if ( $file_data = $item->get_meta( '_img_file' ) ) {
                echo '<p>
                    <a href="'.$file_data['guid'].'" target="_blank" class="button">'.__("Download Image") . '</a><br>
                    <pre><code style="font-size:12px; background-color:#eee; padding:5px;">'.$file_data['guid'].'</code></pre>
                </p><br>';
            }
        }
    }
}

Source: https://stackoverflow.com/questions/62847710/add-image-file-upload-field-to-woocommerce-single-products