It's a pain in the ass. You have to really want to do it in order to do it

1. Type the markdown for an <img> which is ![]. You can optionally put text in between the square brackets for alt text, like this: ![Nitori thinking smugly]
2. Copy the link from the submission image of your choice which should look like https://jstpst.net/submission_images/<random hash goes here>.<jpg/png/gif>.
3. Paste it inside parentheses which will be besides the markdown you did earlier. Like this: ![Nitori thinking smugly](https://jstpst.net/submission_images/<random hash goes here>.<jpg/png/gif>). You can shorten the url a bit by removing the https://jstpst.net; you will get a relative link instead which is nicer.

Now you should get something like this: ![Smug Nitori speech bubble](/submission_images/7f20f25d80863efad8f2600acb32fdaa9d4c50307ff74a1d517f42a7285cda77.jpg) which results in this:

Optionally you can make an inline image a clickable link too; just enclose the whole thing inside another square bracket like you would do for a link with another parentheses outside said square brackets. I do this always so my embeds can link back to the original post (so they can be upvoted that other people can easily embed it too). You can see this with my Mokou heart embed which has this code: [![mokou heart](/submission_images/1a9f50a4b0ee2d39db846d2ef6c48a19596713b0add3e6446f14b248eb0655a7.png)](/f/2hu/10304)

And looks like this: