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.
More so, if it is easily sandboxed, it should just be a webapp. Which discord already is.
Just use the website.
Browsers are already easily themed, have plenty of tools to change deeper functionality, and are way more sandboxed than any other app packaging ecosystem.