Simply installing x11/xpra yields the following result when starting xpra:

[exil@lennon ~]$ more .xpra/lennon-9.log 
..
Xlib:  extension "RANDR" missing on display ":9.0".
Xlib:  extension "RANDR" missing on display ":9.0".
Traceback (most recent call last):
  File "/usr/X11R6/bin/xpra", line 6, in <module>
    xpra.scripts.main.main(__file__, sys.argv)
  File "/usr/local/lib/python2.7/site-packages/xpra/scripts/main.py", line 126, in main
    run_server(parser, options, mode, script_file, args)
  File "/usr/local/lib/python2.7/site-packages/xpra/scripts/server.py", line 304, in run_server
    import xpra.server
  File "/usr/local/lib/python2.7/site-packages/xpra/server.py", line 19, in <module>
    import Image
ImportError: No module named Image
xpra: Fatal IO error 35 (Resursen är tillfälligt otillgänglig) on X server :9.0.
---

After installing graphics/py-imaging, on the other hand:

Xlib:  extension "RANDR" missing on display ":9.0".
Xlib:  extension "RANDR" missing on display ":9.0".
X Error of failed request:  BadValue (integer parameter out of range for operation)
  Major opcode of failed request:  118 (X_SetModifierMapping)
  Value in failed request:  0x17
  Serial number of failed request:  21
  Current serial number in output stream:  21

xpra is ready.
---

Fix: 

Makefile patch:
22a23
>               ${PYTHON_SITELIBDIR}/PIL/Image.py:${PORTSDIR}/graphics/py-imaging \
How-To-Repeat: If necessary, deinstall graphics/py-imaging. Then cd to x11/xpra and make install.