The syntax for an externally hosted image is ![my-alt-text](https://example.com/my.webp). You can put multiple of those in either a post or comment.

I haven’t done it with uploading multiple images to the instance, but I believe you can just keep clicking the add image button. It doesn’t do any fancy previews like the single field in the post for an image, and if you want a preview you have to put one there and more in the body. It’s not a great system, but it works sort of ok.