var randnum = Math.random();
var inum = 7;
// Change this number to the number of images you are using.
var rand1 = Math.round(randnum * (inum-1)) + 1;
images = new Array
images[1] = "imatges/capcalera_1.jpg"
images[2] = "imatges/capcalera_2.jpg"
images[3] = "imatges/capcalera_3.jpg"
images[4] = "imatges/capcalera_4.jpg"
images[5] = "imatges/capcalera_5.jpg"
images[6] = "imatges/capcalera_6.jpg"
images[7] = "imatges/capcalera_7.jpg"

// Ensure you have an array item for every image you are using.
var image = images[rand1]