var imgs = ["img1", "img2", "img3", "img4"];
var tef  = 2;

function start() {
for (i = 1; i < imgs.length; i++) {
var id1 = "img" + i;
var id2 = "img" + (i+1);
var dt  = 2 + (i - 1)*5;
new Effect.Fade(id1, {delay:dt, duration:tef});
new Effect.Appear(id2, {delay:dt, duration:tef});
}
}