Can I put a function directly on a button control?

I am making a slideshow viewer.
Thumnails are displayed as follows. When a user clicks on a thumnail, the large image appear. In img folder, there are 1 thumnail and 1 large image per image, named, for example, 1thum.jpg and 1larg.jpg
<asp:ImageButton ID="img_1" runat="server" ImageUrl="img/1thum.jpg" onclick="img_1_Click" />
<asp:ImageButton ID="img_2" runat="server" ImageUrl="img/2thum.jpg" onclick="img_2_Click" />
<asp:ImageButton ID="img_3" runat="server" ImageUrl="img/3thum.jpg" onclick="img_3_Click" />
When the image button is clicked, below command is triggered.
Protected Sub img_1_Click(ByVal sender As Object, ByVal e As System.Web.UI.ImageClickEventArgs)
   swapImg(1)
End Sub
Then im ...