function image() {
};

image = new image();
number = 0;

// imageArray
image[number++] = "<img src='images/fotos/1.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/2.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/3.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/4.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/5.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/6.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/7.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/8.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/9.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/10.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/11.jpg' border='0' alt='Ceras San Juan'>"
image[number++] = "<img src='images/fotos/12.jpg' border='0' alt='Ceras San Juan'>"
// keep adding items here...

increment = Math.floor(Math.random() * number);

document.write(image[increment]);