PNG 500 error - Save operation failed
If other image formats are working fine, then maybe check that you have Imagick installed and enabled on the server.

Eg.

sudo apt-get install imagemagick php5-imagick
sudo service apache2 reload


If you're on a shared host then either consult their knowledge base or contact support to check for Imagick availability.

Some servers/hosts only have the GD library installed by default, which isn't so good with PNG files.