var posicion; 
//numero aleatorio entre 0 y 21 (si pongo *28 es entre 0 y 26)
posicion = Math.round(Math.random()*22);

images=new Array(28); 
images[0]=new Image(); 
images[0].src="imagenes/logo.jpg"; 
images[1]=new Image(); 
images[1].src="imagenes/logo1.jpg"; 
images[2]=new Image(); 
images[2].src="imagenes/logo2.jpg"; 
images[3]=new Image(); 
images[3].src="imagenes/logo3.jpg"; 
images[4]=new Image(); 
images[4].src="imagenes/logo5.jpg"; 
images[5]=new Image(); 
images[5].src="imagenes/logo6.jpg"; 
images[6]=new Image(); 
images[6].src="imagenes/logo7.jpg"; 
images[7]=new Image(); 
images[7].src="imagenes/logo8.jpg"; 
images[8]=new Image(); 
images[8].src="imagenes/logo14.jpg"; 
images[9]=new Image(); 
images[9].src="imagenes/logo15.jpg"; 
images[10]=new Image(); 
images[10].src="imagenes/logo16.jpg"; 
images[11]=new Image(); 
images[11].src="imagenes/logo17.jpg"; 
images[12]=new Image(); 
images[12].src="imagenes/logo19.jpg"; 
images[13]=new Image(); 
images[13].src="imagenes/logo20.jpg"; 
images[14]=new Image(); 
images[14].src="imagenes/logo21.jpg"; 
images[15]=new Image(); 
images[15].src="imagenes/logo22.jpg"; 
images[16]=new Image(); 
images[16].src="imagenes/logo23.jpg"; 
images[17]=new Image(); 
images[17].src="imagenes/logo24.jpg"; 
images[18]=new Image(); 
images[18].src="imagenes/logo25.jpg"; 
images[19]=new Image(); 
images[19].src="imagenes/logo26.jpg"; 
images[20]=new Image(); 
images[20].src="imagenes/logo27.jpg"; 
images[21]=new Image(); 
images[21].src="imagenes/logo28.jpg"; 

function animar() 
{ 
    document.getElementById("image_placeholder").src = images[posicion].src; 
    posicion = posicion + 1;
    if (posicion==22) {
        posicion = 0;
    }
    timeout_id=setTimeout("animar()",3000); 
}