In feedback window the link to the website of that mail client is shown. And you can click on it.
If you want to use a couple of your own icons, you can use a csv file.
How to do this, please read my documentation about customizing dispMUA at http://www.juergen-ernst.de/addons/dispmua.html
Addon Enigmail OR Mnenhy isn't needed anymore to get special agents with X-NewsReader or X-MimeOLE identified correctly. (since version 1.6.0 of dispMUA)