I had the same problem and i had to reinstall pillow and all is dependencies. Some of my symlink where not good.
First you have to make:
brew doctor
And see if you have some errors. Mine was like this :
Warning: Unbrewed dylibs were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected dylibs:
/usr/local/lib/libjpeg.8.dylib
/usr/local/lib/libpng.3.dylib
/usr/local/lib/libpng12.0.dylib
/usr/local/lib/libpng14.14.dylib
Warning: Unbrewed .la files were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected .la files:
/usr/local/lib/libjpeg.la
/usr/local/lib/libpng12.la
/usr/local/lib/libpng14.la
/usr/local/lib/libpng15.la
Warning: Unbrewed .pc files were found in /usr/local/lib/pkgconfig.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected .pc files:
/usr/local/lib/pkgconfig/libpng12.pc
/usr/local/lib/pkgconfig/libpng14.pc
Warning: Unbrewed static libraries were found in /usr/local/lib.
If you didn't put them there on purpose they could cause problems when
building Homebrew formulae, and may need to be deleted.
Unexpected static libraries:
/usr/local/lib/libjpeg.a
/usr/local/lib/libpng12.a
/usr/local/lib/libpng14.a
So i decided to clean all errors and dependencies i have installed:
brew uninstall pillow
brew uninstall libpng
brew uninstall libjpeg
brew uninstall webp
brew uninstall libtiff
brew uninstall littlecms
brew prune => to clean symlink
When my brew doctor was without any errors i just run brew install pillow
and all was working fine.
Hope it's help.