全选代码：
protected void Allchk_Click(object sender, EventArgs e)
    {
        foreach (RepeaterItem rp in Repeater1.Items)
        {
            CheckBox cbox = (CheckBox)rp.FindControl("chkMorePic");
            if (cbox.Checked == false)
            {
                cbox.Checked = true;
            }
            else

            {
                cbox.Checked = false;
            }
        }
    }
取消代码：
protected void rechk_Click(object sender, EventArgs e)
    {
        for (int i = 0; i <= Repeater1.Items.Count - 1; i++)
        {
            CheckBox check = (CheckBox)Repeater1.Items[i].FindControl("chkMorePic");
            check.Checked = false;
        }
    }
我看你代码中并没有反选按钮么，如果要实现反选功能的话代码如下：
protected void Button1_Click(object sender, EventArgs e)
    {
        foreach (RepeaterItem rp in Repeater1.Items)
        {
            CheckBox cbox = (CheckBox)rp.FindControl("chkMorePic");
            if (cbox.Checked == true)
            {
                cbox.Checked = false;
            }
            else
            {
                cbox.Checked = true;
            }
        }
      
    }