function check(e) {
    var target = (e && e.target) || (event && event.srcElement);
    var curPoint=checkForImage(target);
    if(curPoint==true) {
        document.oncontextmenu=new Function("return false");
    } else {
        document.oncontextmenu=new Function("return true");
    }
}

function checkForImage(t){
    if(t.tagName=="IMG") {
        return true;
    } else {
        return false;
    }
}
document.onmousedown=check;