[R-SIG-Mac] Problem with X11

[Anna Buschart]

Dear R- and MacUsers,

may be a totally disqualifying question, but I am stuck, because: I  
keep getting an Error about an X11 module, for example this:

Error in png(file = namepng, h = h * dpi, w = w * dpi) :
   X11 module cannot be loaded
In addition: Warning message:
In png(file = namepng, h = h * dpi, w = w * dpi) :
   unable to load shared library '/Library/Frameworks/R.framework/ 
Resources/modules/i386/R_X11.so':
   dlopen(/Library/Frameworks/R.framework/Resources/modules/i386/ 
R_X11.so, 6): Library not loaded: /usr/X11/lib/libpng12.0.dylib
   Referenced from: /Library/Frameworks/R.framework/Resources/modules/ 
i386/R_X11.so
   Reason: Incompatible library version: R_X11.so requires version  
42.0.0 or later, but libpng12.0.dylib provides version 38.0.0

The problem only appeared recently (maybe after changing to 2.11.0,  
but I am not sure).

What do I have to do to get the required version of this library?

Thanks in advance,

Anna