you can create like this....
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:scriptmanager id="ScriptManager" runat="server">
</asp:scriptmanager>
<asp:updatepanel id="UpdatePanel" runat="server">
<contenttemplate>
<table>
<tr>
<td>
<asp:textbox id="txtImgUrl" runat="server"></asp:textbox>
</td>
<td>
<asp:button id="btnShow" runat="server" text="Show" onclick="btnShow_Click" />
</td>
</tr>
<tr>
<td>
<asp:label id="lblMessage" runat="server" visible="false"></asp:label>
</td>
</tr>
</table>
<br />
<div>
<asp:image id="imgView" runat="server" alternatetext="Image Goes Here" visible="false" />
</div>
</contenttemplate>
</asp:updatepanel>
</div>
</form>
</body>
</html>
and code behind like...
protected void btnShow_Click(object sender, EventArgs e)
{
if (!string.IsNullOrEmpty(txtImgUrl.Text))
{
imgView.ImageUrl = txtImgUrl.Text;
imgView.Visible = true;
}
else
{
lblMessage.Visible = true;
lblMessage.Text = "Please add Url to text box";
}
}
and use image url like this...
https://www.google.co.in/images/srpr/logo11w.png[
^]