Thank you for help, and tips. I have found a solution.
Here is an aspx code for Gridview field:
<asp:TemplateField>
<ItemTemplate>
<asp:Image ID="Image1" runat="server" ImageUrl='<%# Eval("jan1", "~{0}.bmp") %>' />
</ItemTemplate>
</asp:TemplateField>
And, vb.net code:
Dim img As Image = CType(GridView1.Rows(0).FindControl("image1"), Image)
TextBox1.Text = img.ImageUrl