actually, i think you hand in a handle to the function (not a string) and I don't think you can hand in parameters into the function. Thus, check this out to help understand how you can still pass in parameters to an onclick.
function myOnclick() {
sendingObj = event.srcElement;
alert (sendingObj.myProperty);
}
...
img.myProperty = "blahblah";
img.attachEvent("onclick", myOnclick); //notice, no brackets... no parametes, no quotes