Web Forms :: ImageButton - Onclick Doesn't Require 2 Parameters?

Why isn't the image displayed? The folder images is in the same folder as the .aspx file.Why doesn't it invoke the ImageButton1_Click method when I click on the image? The OnClick should be for the server side.The ImageButton1_Click method takes 2 parameters, but why OnClick does not require 2 parameters?

<asp:ImageButton
ID="ImageButton1"
runat="server"
ImageUrl="images/export.gif"
OnClick="ImageButton1_Click"
/>
protected void ImageButton1_Click(object sender, ImageClickEventArgs e)
{
Label1.Text = "You've clicked the image button.";
}

I have to add this line to the Page_Load method in order for it to invoke the ImageButton1_Click method when I click on the image, why?

ImageButton1.Attributes.Add("onclick",
"ImageButton1_Click()");

This line works fine too:

ImageButton1.Attributes.Add("onclick",
"ImageButton1_Click('report.xls', GridView1)");

The 2 parameters are: object sender, ImageClickEventArgs e. What are they and why I pass in a string and a gridview, which I don't think match with the arguments definition, and it still works?