var anzBilder = 18;   
var Bilder = new Array(anzBilder-1);   

Bilder[0] = new Array("header-pic.jpg","");   
Bilder[1] = new Array("header-pic1.jpg","");
Bilder[2] = new Array("header-pic2.jpg","");
Bilder[3] = new Array("header-pic3.jpg","");
Bilder[4] = new Array("header-pic4.jpg","");
Bilder[5] = new Array("header-pic5.jpg","");
Bilder[6] = new Array("header-pic6.jpg","");
Bilder[7] = new Array("header-pic7.jpg","");
Bilder[8] = new Array("header-pic8.jpg","");
Bilder[9] = new Array("header-pic9.jpg","");
Bilder[10] = new Array("header-pic10.jpg","");
Bilder[11] = new Array("header-pic11.jpg","");
Bilder[12] = new Array("header-pic12.jpg","");
Bilder[13] = new Array("header-pic13.jpg","");
Bilder[14] = new Array("header-pic14.jpg","");
Bilder[15] = new Array("header-pic15.jpg","");
Bilder[16] = new Array("header-pic16.jpg","");
Bilder[17] = new Array("header-pic17.jpg","");
Bilder[18] = new Array("header-pic18.jpg","");

var Zahl = Math.random()*anzBilder
var j = Math.floor(Zahl);

html = "";
html += "<img src=\""+Bilder[j][0]+"\" alt=\""+Bilder[j][1]+"\">";
document.write (html);