If you get a white screen when exporting an ePub with PNG on OS X Yosemite...

[Joseph Flowers]

This is mostly just a heads up in case anyone else runs into the same problem.  When exporting ePubs that included a PNG image, I kept getting a blank white screen, despite having WP_DEBUG enabled.  It was only happening when testing locally on my laptop and not on our hosted instance.  It turns out that this is because the version of PHP that comes with OS X Yosemite doesn't come with 'imagecreatefrompng' as part of the distribution.

I traced it to the following line, which has an @ to suppress the output of the message, but once removed, it is throwing a Fatal Error when I try to export an ePub with a PNG, hence the white screen.  

https://github.com/pressbooks/pressbooks/blob/dev/includes/pb-image.php#L74

Discussion of the issue:

https://discussions.apple.com/thread/6617744