Sorry I was wondering if you’ve seen a live example of this being done. I assume the screenshot is photoshopped?
I’m not aware of a good way to add a png icon before headings automatically unfortunately – it’s usually done with font icon or some sort.
You can add it in manually like this though:
<img src="https://tinytodolist.com/wp-content/uploads/2019/12/Arrowicon.png class="heading-icon"><h3>Title Text</h3>
If you can find an actual/live example of this png icon being done with CSS, I can likely tell you how they did it.