Try this -

In ascx mode:
<dnn:LOGIN runat="server" id="dnnLOGIN" Text="<img src=&quot;LOGIN.gif&quot;>" LogoffText="<img src=&quot;Logout.gif&quot;>" /> 

Or in XML:
<Object>
<Token>[LOGIN]</Token>
<Settings>
<Setting>
<Name>Text</Name>
<Value><![CDATA[<img src="LOGIN.gif">]]> </Value>
</Setting>
<Setting>
<Name>LogoffText</Name>
<Value><![CDATA[<img src="Logoff.gif">]]> </Value>
</Setting>
</Settings>
</Object>

This worked fine in DNN version 2, you may need to add <%= SkinPath %> in the ascx for the later versions.