Change your query:
const string query = "select SID, SName, IsNull(NullIf(Image, ''), @DefaultImage) Image from Students";
SqlDataAdapter sda = new SqlDataAdapter(query, con);
sda.SelectCommand.Parameters.AddWithValue("@DefaultImage", "~/images/no-picture.jpg");
Specify the path of your default picture as the value of the
@DefaultImage
parameter.