@wuff
You can also post your image using the image bbcode tags like so:
[img]https://userimages-akm.imvu.com/userdata/47/57/76/12/userpics/Snap_pMMoQhhwMr381908456.jpg[/img]
Just make sure it's a direct link, meaning you can see the extension, in this case .jpg.
As
@Soli
said, you can click the preview button to go to the full post editor (the one you see at the bottom of topics is a quick post editor, not the full one). The full post editor has more features/functionality and it meant for longer more thought out posts.