function roll_over(img_name,img_src)
{
	document[img_name].src = img_src;
}

function preload_menu()
{
	// preload menu images
	m_porexpan_o = new Image(); 
	m_porexpan_o.src = "img/m_porexpan_o.png";
	m_produccion_o = new Image(); 
	m_produccion_o.src = "img/m_produccion_o.png";
	m_galeria_o = new Image(); 
	m_galeria_o.src = "img/m_galeria_o.png";
	m_contacto_o = new Image(); 
	m_contacto_o.src = "img/m_contacto_o.png";
	m_links_o = new Image(); 
	m_links_o.src = "img/m_links_o.png";
}