PWA: service-worker won't precache my images

I’m trying PWA for the first time. I’ve overcome the first problem of precacheing my lazy loaded modules by putting

   IonicModule.forRoot(MyApp,  {  preloadModules: true})

in the app/module.ts, but I can’t seem to find out how to do the same with all the images and other static assets.
There’s a line in service-worker.js:-

self.toolbox.precache( [  [

so I thought it would go there, I tried ‘./assets/img/myimage.png’ and ‘/assets/img/myimage.png’ and ‘assets/img/myimage.png’ and various * too, but nothing gets precached - I’m checking that by looking in the sources of chrome dev tools.

Have I overlooked something trivial here?