I've discovered that I can record my iphone screen!!! So I've just done this very quick video showing how easy it is in the ColorScape app to create an outline.
You literally open the app, choose a photo from your camera roll, it immediately changes it to an outline, and then you can tweak the resolution and details etc. It does cost around £5 but it's totally worth it and it does it all for you - not messing about with photoshop gimp :)