I have just added a new theme component which now allows you to annotate your images within the topic compose!
Just upload your image, and click it on the preview; a new toolbar will appear:
You can add rectangles, highlight or insert text…
Just click the to save your changes! Your new image will be published
P.S.: This theme component is (alpha) Image Annotator - Annotate images right inside the previewer! - theme - Discourse Meta
It is powered by GitHub - ailon/markerjs: Let your users annotate and mark images