How are you populating the page...?

<%# is databinding syntax - if you just want to refer to the text in the
textbox, try:

<%=textbox1.txt%>

However, depending on how you are fetching / populating the initial data, it
may not be available when the control is rendered...

You can keep your syntax if you wish, just add runat=server and call
DataBind method for the page.

Or you can use <%= syntax:
<a href='<%= textbox1.text %>'>

Or you can use server controls HyperLink and Image and set their attributes
programmatically.

why would you use a textbox as an image?

why not just use the <img> tag?

I'm doing something like this to populate an image based on what is returned from the db.

public string GetDocImage(object docName)
   {
       string strImg;
       if(docName== DBNull.Value)
       {
           strImg = "~/images/empty.gif";
       }
       else
       {
           strImg = "~/images/Docname" + docName+ ".gif";
       }
       return strImg;
   }

and in the HTML, I do this:

<asp:Image ID="imgDoc" runat="server" AlternateText='<%# Eval("DocName%>'
                                                   ImageUrl='<%# GetDocImage(Eval("docName")) %>' />

this returns the docName and the URL for the document to open