ImageUrl - dynamic value?

Hi,
The net seems to not have a solution to my problem.
I have an ImageButton where I need to assign the value of ImageUrl dynamically, because the folder/path is set in code-behind during runtime. I've tried everything that I've found but nothing seem to work. Is this even possible?
<asp:ImageButton ID="ImageButton1" runat="server" ImageUrl='<%= GetImageHere("test.jpg") %>' />
in module function (vb.net)
public function GetImageHere(ByVal myImage as string) as string
return "/Folder1/" & dynamicFolderName & "/Folder2/" & myImage
end function
Would really need a bit of help, and much appreciated. Thanks
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