Install png on yosemite & RRO - r

Question

I am trying to install the package png on a macbookpro 2014 & yosemite.
I am using RRO (but I believe I would have a similar issue with R).
This is what I can see on the RRO console:
* installing *source* package ‘png’ ...
** package ‘png’ successfully unpacked and MD5 sums checked
** libs
clang -I/Library/Frameworks/R.framework/Resources/include -DNDEBUG -I/usr/local/include -I/usr/local/include/freetype2 -I/opt/X11/include `libpng-config --cflags` -fPIC -Wall -mtune=core2 -g -O2 -c read.c -o read.o
/bin/sh: libpng-config: command not found
clang -I/Library/Frameworks/R.framework/Resources/include -DNDEBUG -I/usr/local/include -I/usr/local/include/freetype2 -I/opt/X11/include `libpng-config --cflags` -fPIC -Wall -mtune=core2 -g -O2 -c write.c -o write.o
/bin/sh: libpng-config: command not found
/bin/sh: libpng-config: command not found
clang -dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/usr/local/lib -o png.so read.o write.o -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation
/bin/sh: libpng-config: command not found
installing to /Library/Frameworks/R.framework/Versions/3.1/Resources/library/png/libs
** R
** inst
** preparing package for lazy loading
** help
*** installing help indices
** building package indices
** testing if installed package can be loaded
Error in dyn.load(file, DLLpath = DLLpath, ...) :
unable to load shared object '/Library/Frameworks/R.framework/Versions/3.1/Resources/library/png/libs/png.so':
dlopen(/Library/Frameworks/R.framework/Versions/3.1/Resources/library/png/libs/png.so, 6): Symbol not found: _png_create_info_struct
Referenced from: /Library/Frameworks/R.framework/Versions/3.1/Resources/library/png/libs/png.so
Expected in: flat namespace
in /Library/Frameworks/R.framework/Versions/3.1/Resources/library/png/libs/png.so
Error: loading failed
Execution halted
ERROR: loading failed
On the other hand I have installed (using homebrew) libpng and I can type libpng-config on the terminal having a response
Usage: /usr/local/bin/libpng-config [OPTION] ...
I am running out of options...