[Zope] if text: display text else icon with tal

I have a problem with my Page Template: I want the visitors to have the opportunity to get one level up. In my product I have a field that is called 'level_up_txt'. If this field is filled in I want to use that, but when they leave the field empty they have to get a levelup icon. How can I arrange this in TAL? I've tried it like this: <a class="gnawlink" href="" tal:attributes="href container/absolute_url" tal:content="container/level_up_txt"> <img src="icon" width="16" height="16" border="0" alt="item" tal:attributes="src here/upfolder; alt here/meta_type" /></a> Where upfolder is the icon that I want to use. Can anybody help me? Greetz Nico