When you post it the image should show up, even if it just looks like a link when typing the comment
In markdown, there is the notation []() for links. Reddit allowed it too for examples, and generally a lot of programs and platforms that have mild text formatting use markdown.
[some text](https://example.org/some-link) will turn into some text
Lemmy has basically extended this with ![]() which shows the content of the link
![some text](https://upload.wikimedia.org/wikipedia/commons/7/70/Example.png) will turn into
Where did that “some text” go? It’s basically the placeholder for when the image is loading or failed to load, the correct term is the alt-text.
The image @Branch_Ranch@lemmy.world was asking about uses the text
![](https://ttrpg.network/pictrs/image/396cb01b-6b2b-4351-9cd5-0742c2914719.png)
It has no alt text. Any frontent that has an image upload button or similar will upload the image somewhere, take the link, and put it into your post like this.
I hope your frontend renders code-blocks and escapes with backslash (\) correctly, else this may look weird to you.