The normal case was to use the builtin icons. There were two cases
where the icon file was read from disk:

1) /usr/NX/share/images/nxagent.xpm was existing which normally is not
   there

2) case 2 was not true and nxagent.xpm was existing somewhere in the
   PATH (!)

(replace nxagent.xpm by x2goagent.xpm if in x2gp mode)

Scanning the path from the PATH variable for xpm files is kind of
unexpected and dangerous, too (think of automounter triggers or
invalid xpm files).

Also remove the xpm files from the distribution.