try
protected void GridView1_RowCommand(object sender, GridViewCommandEventArgs e)
{
if (e.CommandName == "Select")
{
var index = Convert.ToInt32(e.CommandArgument);
var row = GridView1.Rows[index];
var filePath = row.Cells[2].Text;
var fullFileName = Path.Combine( Server.MapPath( "GalleryFiles") + filePath);
if (File.Exists(fullFileName))
{
Image1.ImageUrl = fullFileName;
}
}
}