Django ImageField "Upload a valid image. The file you uploaded was either not an image or a corrupted image."
This issue was solved by using pip to install "pillow" instead of "pil" which allows easy deployment to virtualenv.
I have PIL installed, however whenever I try to upload a .png file to an image field through the Django Admin for my model, I get this error:
"Upload a valid image. The file you uploaded was either not an image or a corrupted image."
Other image types work fine. I have tried several different PNG files. 
I have tried rebuilding PIL after installing pypng, libpng-dev, etc.. and am on ubuntu server.