Image Loop With Preloader

I want to build a loop, which cycles through a set of images and displays each image for a few seconds. In Firefox everything works fine, IE6 also executes the code but gives an script error in the status bar ("not implemented"). The relevant line of code is in red in the code below.

What could cause this ugly error in IE?

Code:

var imgpath = "imgtest/";
var errpic = 1;
var maxpic = 5;

function imgloop(actno) {
if(actno==maxpic) {
actno=1;
} else {
++actno;
}
preload(actno);
}

function imgshow(actno) {
document.mainpic.src = imgpath + actno + ".jpg";
imgloop(actno);
}

function delay(actno) {
setTimeout("imgshow(" + actno + ")", 5000);
}

function preload(actno) {
img = new Image();
img.onload = delay(actno);
img.src = imgpath + actno + ".jpg";
}