Right now only link-only messages can get oneboxed into 300×300px images. If you add any text before or after an image link, it's not oneboxed. The main site markdown:
![fascinating optics phenomenon](https://i.stack.imgur.com/asdfasd.png) also doesn't work. But it most definitely should, because right now any starred image onebox appears as
https://i.stack.imgur.com/asdfasd.png and not
fascinating optics phenomenon. In chat, the description text would appear below the image.
Alternatively, image link is on the first line, followed by a new line (Shift+Enter) and the rest should be the description text. Of course, new lines are removed when seen in starred list, but in chat, the image can have a nice description that is not limited to desktop users (on hover).