var current = 0;

function initArray() {

this.length = initArray.arguments.length;
  for (var i = 0; i < this.length; i++) {
  this[i] = initArray.arguments[i];
  }
}

var picz = new initArray(

"p1.gif",
"p2.gif",
"p3.gif",
"p4.gif",
"p5.gif",
"p6.gif",
"p7.gif"
);

  document.write('<img name="jscriptimg" src="'+picz[0]+'">');

function checkIt(val) {

var goodnum = current+val;

if (goodnum < 0) alert("Anfang!");
else if (goodnum > picz.length-1) alert("Ende!");
else document.jscriptimg.src = picz[goodnum];

current = goodnum;
}