Great, now ImageJ wiki pages contain some Open Graph metadata like this:
<meta property="og:type" content="website"/>
<meta property="og:title" content="ImageJ"/>
<meta property="og:image" content="http://imagej.net/skins/imagej-128.png"/>
<meta property="og:url" content="http://imagej.net/Welcome"/>
However, oneboxing of wiki pages in Discourse still doesn't work yet (as opposed to Google or Facebook, see this tool), because I missed the fact that it also requires a
description tag to be present.
Consequently I suggest to also install the Description2 extension which automatically extracts a description from the page content and also allows to explicitly specify a description using a tag. I opened a pull request for this: