Just checked again with jpegs to be certain, and it still works for me on on Windows (bundled Pillow v5.4.1) and on Linux (system Pillow v6.0.0).
I'd focus on making sure the following works in general on the Macs in question (with the bundled version of Pillow):
Code:
from PIL import Image, ImageTk
image = Image.open("image.jpg")
photo = ImageTk.PhotoImage(image)