Like this ?

When you reply, in the markdown editor there is an image button. You may have to go in settings to make the markdows editor appear.

I'm trying to upload an image, but the upload is failing. Can't figure out why !!

Ah that you should ask the sysadmin of your instance @jerry@feddit.online :)

Here's how you make links in general.
[text goes here](https://site.tld/stuff/image.png)

Add an exclamation mark if you want to turn the link into a picture.
![This makes the picture visible without clicking it.](https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Cone-fundamentals-with-srgb-spectrum.svg/960px-Cone-fundamentals-with-srgb-spectrum.svg.png)

Here's how that looks like in practice.

Figured it out myself :)))

OK, so please tell the rest of us what you learned, so that we who've never done it can understand what's required?

_ /\ _

I simply moved the image to a different folder, from which it got picked up by the server.