I added another drawing app to my phone just before I left so that I could try it out on the flight. It's called Pocket Sketchbook and this was the free version. It didn't have layers so, although I was able to load a photograph, I had to alter the photograph itself. I found the drawing options quite limiting and the drawings I finally came up with weren't very good. We visited Singapore and I tried to create a simple skyscraper silhouette for manipulation. I also coloured over a lily that I had on the phone.
This the skyscraper:
The only thing I managed to come up with was a pattern. Nothing else I seemed to do to it looked like anything special.
This is what I did to the lily: