You could try something like:

def addtopage(function):
   png_count = 0
   start_y = 20
   img_ht = 90
   for f in os.listdir('path'):
      if f.endswith('png') in f:
         if png_count%2 == 0
             pdf.image(f, 5, start_y, WIDTH/2-10)
         else:
             pdf.image(f, WIDTH/2-10, start_y, WIDTH/2-10)
             start_y += img_ht
         png_count += 1
         if png_count%6 == 0:
             pdf.add_page()

You could still improve this code and have some more variables that will allow for more flexibility, but this should give a good boilerplate for the same. Also, if you are using ppyfpdf, I would suggest taking a look at fpdf2. It is a fork from PyFPDF itself and is being actively maintained (PyFPDF received its last commit in 2018).