I might have been too hasty, there is a problem. At the drilldown level the display is fine, when sitting in the subcategory you can see the correct gravatar and short name “kiwib”: http://www.engdex.pl/forum/engdex/kbase/
But looking from the top level, it seems to be showing my gravatar, but with a different user name, “akwadmin”. http://www.engdex.pl/forums/
For us numberless masses that want to implement exactly what you have, i.e. displaying nickname instead of fullname, without learning the whole product, could this simply be a procedure?
1: Edit file xxx and replace with these lines:
2: Edit file ABC and replace DEF with GHI