Converts a multi-page pdfs to a list of pngs via the `sips` command
  :returns: A list of converted pngs

def to_pngs(pdf_path):
  ''' Converts a multi-page pdfs to a list of pngs via the `sips` command
  :returns: A list of converted pngs
  '''
  pdf_list = split_pdf(pdf_path)
  pngs = []
  for pdf in pdf_list:
    pngs.append(to_png(pdf))
    os.remove(pdf)  # Clean up
  return pngs