Monogame on Mac has a bug (as of 1/12/2020) where libfreeimage.dylib is not found during build. To fix this, do the following:
brew install freeimage
# find the installed `libfreeimage.dylib`:
brew ls freeimage | grep -e 'libfreeimage\..*\.dylib'
# link it into the MG builder build tool folder:
ln -s \
/usr/local/Cellar/freeimage/3.18.0/lib/libfreeimage.3.18.0.dylib \
~/.nuget/packages/monogame.content.builder/3.7.0.4/build/MGCB/build/libfreeimage.dylib
For detailed setup step: http://kflu.github.io/2020/01/12/2020-01-12-monogame-on-mac/