I found that the "best" way is to use HTML, as it works both in Readme/.md files and also in comments (within Issues, Gist...)
E.g. when adding/editing a comment (within Issues, Gist...) :
- Upload the picture by drag-and-drop in the text field
- replace
with<img src="https://your-image-url.type" width="100" height="100">
As mentioned by @cbestow (thanks!), it's not mandatory to set both width and height. If only one is set, the other will be adjusted accordingly to preserve the aspect ratio of the image.

Really helpful! Thanks