<asp:Image ID="Image1" runat="server" ImageUrl="~/asp-net.jpg" />

ToolTip is an important property in image server control. When mouse will go over the image then Text will be displayed which I have written in ToolTip.

<asp:Image ID="Image1" runat="server" ImageUrl="~/asp-net.jpg" 
ToolTip="WELCOME ASP.NET " />