// Set header style
function myprefix_search_page_header_style( $style ) {
	if ( is_search() ) {
		$style = 'background-image';
	}
	return $style;
}
add_filter( 'wpex_page_header_style', 'myprefix_search_page_header_style' );

// Adds page header bg
function myprefix_search_page_header_bg( $image ) {
	if ( is_search() ) {
		$image = 'YOUR IMAGE URL';
	}
	return $image;
}
add_filter( 'wpex_page_header_background_image', 'myprefix_search_page_header_bg' );