Ya the mozilla docs are always a good place to start: https://developer.mozilla.org/en-US/docs/Web/API/Canvas_API
That will probably get you everything you need to get started, just follow through each section listed in the sidebar:
You can see it covers drawing shapes, using images, and hit regions. Which together with some touch position code inside the canvas should give you everything you need, except the actual user interface you want to build with it.
You’d also need the ionic native image picker or camera plugin to load the image itself, but after that it should all work the same whether it’s ionic or just a responsive website.