Some info to help people debug this:

I have attached the logo file, x2go.xpm

It was introduced on 2012-01-11 as part of a patch file in this commit:
http://code.x2go.org/gitweb?p=nx-libs.git;a=commitdiff;h=405bc8f259f230947e5fe8341f0bab8f0f88c678

That patch file has since been renamed 2 times, but it the logo has
never changed.

It is displayed by nx-X11/programs/Xserver/hw/nxagent/Display.c :
http://code.x2go.org/gitweb?p=nx-libs.git;a=blob;f=nx-X11/programs/Xserver/hw/nxagent/Display.c;hb=refs/heads/3.5.0.x

which we apply the aforementioned patch to:
http://code.x2go.org/gitweb?p=nx-libs.git;a=blob;f=debian/patches/0201_nxagent_set-x2go-icon-if-x2goagent-flavour.full.patch;hb=refs/heads/3.5.0.x

-Mike#2