I have a strange behavior with Drupal. When I load a PNG in a image field it does upload the image but does not resize it. The strange thing is that it works well with JPG.
I did :
chmod 777 files/
and
chmod 775 files/
but did not change anything.
I am under : OSX Yosemite 10.10.1, PHP 5.5.18 and apache 2.4.9
Once upgraded to Yosemite, PHP don't handle anymore PNG via GD. So we have to install a new version of php with homebrew like explain here : https://stackoverflow.com/a/26505558/3656447