将母版页ImageButton事件传递到内容页

public event EventHandler SomethingHappened;

protected void Button_Click(object sender, EventArgs e)
{
    OnSomethingHappened(EventArgs.Empty);
}

protected void OnSomethingHappened(EventArgs e)
{
    if(this.SomethingHappened != null)
    {
        this.SomethingHappened(this, e);
    }
}

protected override void OnInit(EventArgs e)
{
    base.OnInit(e);
    //allows us to change master pages
    if(this.Master is MyMaster)
    {
        ((MyMaster)this.Master).SomethingHappened += new EventHandler(HandleSomethingHappened);
    }
}

private void HandleSomethingHappened(object sender, EventArgs e)
{
    //deal with it
}

MSDN reference

what Rex M specified

// Master Page Code
public event EventHandler ClearClick
{
   add
   {
      this.btnClear.Click += value;
   }
   remove
   {
      this.btnClear.Click -= value;
   }
}

// Content Page markup
<%@ Page  masterPageFile="~/MasterPage.master"%>

<%@ MasterType  virtualPath="~/MasterPage.master"%> 

// Content Page Code
protected override void OnInit(EventArgs e)
{
    base.OnInit(e);
    this.Master.ClearClick += new EventHandler(OnClearClick);
}

private void OnClearClick(object sender, EventArgs e)
{
   // Handle click here
}