Won't show in the preview, correct.
To make it show in the body of the text, upload it as before, then you can copy the link to that image (now it's stored on the forum server), and then edit the post and add an image using Insert Image, pointing to the the link you saved. You could also in theory upload the image to another server of your own choosing, then use Insert Image and point to that (as a note, for the Gallery, if it isn't hosted on our servers then by and large it won't be shared on our socials, so uploading it to the forum as an attachment then editing the post to embed the image is the preferred method there to keep you in the running).
