You can set the visible property to false, like so:
<asp:Image ID="imageName" runat="server" Visible="false" ImageUrl="yourURL"/>
Then in your MasterPage.cs set visible if in role:
if (HttpContext.Current.User.IsInRole("Admin"))
{
imageName.Visible = true;
}
else
{
imageName.Visible = false;
}
This is the easiest approach to me.