add_action('cred_form_validate', 'my_image_validation_func',10,2);
function my_image_validation_func($error_fields, $form_data) {
    // all the valid file types
    $file_types = array('image/jpeg','image/png','image/jpg');
    $max_size = 5 * 1024 * 1024;
    $forms = array( 1806 );
 
    // Field data are field values and errors
    list($fields,$errors)=$error_fields;
 
    if (in_array($form_data['id'], $forms ) && (isset($_FILES['wpcf-book-images']['type']))) {
        // 1. Minimum number of photos is '3'.
        if ( count( $_FILES['wpcf-book-images']['name'] ) < 3 ) {
            $errors['wpcf-book-images'] = 'Add at least 3 images';
            return array( $fields, $errors );
        }
 
        // 2. Dimensions of the photo should be at least 350 (height) x 350 (width).
        foreach ( $_FILES['wpcf-book-images']['tmp_name'] as $tmp_image ) {
            $sizes = getimagesize( $tmp_image );
            if ( $sizes[0] < 350 || $sizes[1] < 350 ) {
                $errors['wpcf-book-images'] = 'An image is too small. Images should be at least 350x350';
                return array( $fields, $errors );
            }
        }
 
        // 3. The file types should be JPG, PNG and JPEG.
        foreach ( $_FILES['wpcf-book-images']['type'] as $type ) {
            if ( !in_array( $type, $file_types ) ) {
                $errors['wpcf-book-images'] = 'Image file type is not supported: ' . $type;
                return array( $fields, $errors );
            }
        }
 
        // 4. Maximum file size should be 5MB.
        foreach ( $_FILES['wpcf-book-images']['size'] as $size ) {
            if ( $size >= $max_size ) {
                $errors['wpcf-book-images'] = 'An Image is too big. Images should less than 5MB';
                return array( $fields, $errors );
            }
        }
    }
 
    //return result
    return array($fields,$errors);
}