The settings on this message board don’t allow users (any of us, not just new users) to directly upload image files. If you want to embed an image in your post, you have to put the URL of the image from another online site into your post (on its own line of text).
Note that the board’s software (Discourse) will let you go through the process of trying to share an image, before telling you that you can’t when you try to post it. It’s not ideal.
If you are trying to share an image from your own computer (such as a screenshot), you’ll have to first upload the image to an image-sharing site, and then share the URL for the image there in your post.