ASP.NET Image Control not updated after ImageUrl change

I'm fairly new to ASP.NET and of course happened to stumble some strange problem.

Problem is very popular, but none of the replies I found worked for me.

I have a simple image:

<asp:Image ID="userImage" runat="server" ImageAlign="Right" />

And in codebehind:

If Not Session("selecteduser") Is Nothing Then
    userImage.ImageUrl = "~/showphoto.aspx?user=" & Session("开发者_高级运维selecteduser")
End If

And the image doesn't change. It doesn't even fire up the showphoto.aspx code. The property is changed (because next time i step into it, it's changed), but showphoto is never fired.

I'm aware of image caching issues, but I can't get even one image, not mentioning have it changed. For sure there's a very simple explanation, but I'm fighting with this for 2 days now to no avail.

I think the problem may be one of the following:

1. There is nothing in the session object "selecteduser" which might cause that the imageurl property not set. ( Resolution: debug your code to confirm that there is a session object )

2. The image url is being setted but the image is not changed which might be a problem of browser caching. (Resolution: Append the current date time to the list of query string parameters to force the browser to load the new image).

3. "showphoto.aspx" is a page and not an image so it return html code which is not translated to image. (Resolution:Change the content-type property of this page to be image