@Trüff said

but I used the class Image from the PIL module instead of the one from Pythonista‘s ui module and that worked

👍

Pythonista is iOS only. The feature are described at http://omz-software.com/pythonista/ and http://omz-software.com/pythonista/docs/ios/ The modules like ui, scene, and canvas allow you to build iOS apps in Python. The inclusion of NumPy, Matplotlib, Requests, BeautifulSoup, Flask, Jinja2, Pillow, etc. are also quite powerful.

Note to self (and others so afflicted)—
Starting with this seemed to fix it:
set_fill_color(1.0,1.0,1.0)
fill_rect(0,0,scrSize[0],scrSize[1])

where i've done this at global scope:
scrSize=(1136,640) # iPhone 5
set_size(*scrSize) # min 16, max 2048

(my canvas is meant for landscape viewing)

https://gist.github.com/389a67c5aacb097b87fd

Simply import this function, and run it after drawing to the canvas. see example.
It is not necessary to fill the canvas white- it is actually transparent, unless you have drawn to it (as should be obvious when you save an image then quickview it)