how to add Image button and text in Gridview header

You can add a template column and add link button to it. If you want an image you can use image button. If you want both image and text, then you can embed images tag inside the link button.
Bind the command argument property with the ID of the item and handle the RowCommand event of the grid view.

Grid view:

<asp:GridView ID="GridView1" runat="server">
<Columns>
<asp:TemplateField HeaderText="Book">
<ItemTemplate>

<asp:LinkButton ID="LinkButton1" runat="server"
CommandArgument='<%# Bind("ID") %>' CommandName="bookitem">
<asp:Image ID="Image1" runat="server" ImageUrl="~/Images/Filter.gif" /> Book this item</asp:LinkButton>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

Code behind:

Protected Sub GridView1_RowCommand(sender As Object, e As System.Web.UI.WebControls.GridViewCommandEventArgs) Handles GridView1.RowCommand
If e.CommandName = "bookitem" Then
'' add code here
'' you can use e.CommandArgument to get the CommandArgument
End If
End Sub

You can use HeaderTemplate and add any control to it...

Just like ItemTemplate used in second solution..

read more about HeaderTemplate Here

<asp:gridview id="GridView1" 
        width="250" 
        runat="server">

        <columns>
          <asp:templatefield>
            
            <headertemplate>
              <asp:ImageButton ID="btn1" runat="server" ImageUrl="img1.gif" OnClick="btn1_Click"/>
            </headertemplate>
          </asp:templatefield>                      
        </columns>

      </asp:gridview>