check this.
DataTable dt = new DataTable();
dt.Columns.Add(new DataColumn("Image", typeof(string)));
DataRow dr;
int i = 0;
foreach (string file in Directory.GetFiles(Server.MapPath(@"ImageUpload\")))
{
dr = dt.NewRow();
dt.Rows.Add(dr);
dr["Image"] = "ImageUpload/" + System.IO.Path.GetFileName(file);
i += 1;
}
GridView1.DataSource = dt;
GridView1.DataBind();
<asp:GridView ID="GridView1" runat="server" AutoGenerateColumns="false">
<Columns>
<asp:ImageField DataImageUrlField="Image">
</asp:ImageField>
</Columns>
</asp:GridView>