<asp:Image ID="Image1" runat="server" Height="20" Width="20" ImageAlign="Bottom" />
<asp:DropDownList AutoPostBack="true" ID="DropDownList1" runat="server"
onselectedindexchanged="DropDownList1_SelectedIndexChanged">
<asp:ListItem>Available</asp:ListItem>
<asp:ListItem>Busy</asp:ListItem>
<asp:ListItem>Invisible</asp:ListItem>
</asp:DropDownList>
Step 2: Implement the DropDown's SelectedIndexChange Event
string status=DropDownList1.SelectedValue.ToString();
if (status == "Available")
{
Image1.ImageUrl = "status/available.jpg";
}
else if (status == "Busy")
{
Image1.ImageUrl = "status/buzy.jpg";
}


hope this will help you!