Should you want to show the name of the last user who published an object on a publishing destination, use such xPath:

nav:referencing/nav:referencingBy[@attrName='pubDest.objs']/nav:refs/nav:refObject/gn4:pubDest/gn4:objs/gn4:ref[fn:objectIdFromString(@idref)=$picId]/gn4:userRef/nav:refObject/gn4:user/@fullName

An alternative is to use the auditing data instead of referencing, e.g:

<ReportOptions

  NavOptions="Names,Audit"

  . . . 

and therefore the XPath becomes:

nav:audit/nav:actionDesc[@Action='Publish']/@UserDesc

or even:

nav:audit/nav:actionDesc[@Action='Publish']/gn4:user/nav:refObject/gn4:user/@fullName