Refer this code

HTML

<asp:FileUpload ID="FileUpload1" onchange="FileChange();" runat="server" />
<div style="display: none;">
    <asp:Button ID="Button1" Text="Save" runat="server" OnClick="Save" />
</div>

JavaScript

<script type="text/javascript">
    function FileChange() {
        document.getElementById("<%=Button1.ClientID %>").click();
    }
</script>

Namespace

using System.IO;

C#

protected void Save(object sender, EventArgs e)
{
    if (this.FileUpload1.HasFile)
    {
        this.FileUpload1.PostedFile.SaveAs(Server.MapPath("~/Images/" + Path.GetFileName(this.FileUpload1.PostedFile.FileName)));
    }
}

You can make use of Uploadify also.

Asynchronous File Upload with jQuery Uploadify in ASP.Net