Try like this :
foreach (GridViewRow gr in GridViewTn.Rows)
{
HyperLink hp = new HyperLink();
hp.Text = gr.Cells[1].Text ;
hp.Attributes.Add("onmouseover", "return showImg('img1.jpg');");
hp.Atrributes.Add("onmouseout", "return HideImg('img1.jpg');");
hp.NavigateUrl = "NarganTnFiles.aspx?Name=" + hp.Text + "-" + gr.Cells[2].Text + gr.Cells[5].Text;
gr.Cells[1].Controls.Add(hp);
}
In aspx javascript
<script language="JavaScript" type="text/javascript">
function showImg(imgname)
{
}
function HideImg(Imagename)
{
}
</script>