Your question is not clear,, but as i understood in two directs,, I am trying here to put solution for both...
If you wana know the asp:image control property where you can define image for control,,,
You can use
ImageUrl="~/Images/abc.gif"
there...
and if you want to know about the place/folder where you want to keep images for your project so for that you can create an ASP.NET folder Themes and than create a folder named Images in your application or you can create a folder whereever in your application and keep the images there but this last option will be not suitable for a Good Programmer.
Hope this will help... :)
EDIT::
Here I have an example..
<asp:Image ID="img1" runat="server" ImageUrl="Place ur image's URL here" />