As far as I can tell, the only images are those that are usual for a category – logo, avatars and so on.
The black rectangle that appeared for https://discourse.opensourcedesign.net/c/feedback was https://pbs.twimg.com/card_img/863280008181239808/vRvsRZj3?format=png&name=600x314
I deleted the tweet where that rectangle was first seen.
There was a suggestion to use http://iframely.com/debug, all three links appear fine, there's the logo:
Further experiments with the Open Source Design forum
The original tweet used all available characters (0 remaining), I wonderered whether that was a factor so I experimented with something shorter:
An experiment with just one link, surrounded by text:
A link – the forum home, https://discourse.opensourcedesign.net/categories – alone:
Experiments wih other instances of Discourse
I experimented with four instances. At a glance, none of the four is affected.
Note the generic icon for a page:
Not long after posting, Twitter begins to show a forum-specific image:
Something like the tweet in the opening post – three links, and the maximum number of characters:
Back to Firefox, where I made the first tweet about Mozilla Discourse:
After a simple reload of the page, there's the forum-specific image in lieu of the generic icon for a page:
As I have not yet reproduced the issue with any other instance of Discourse, I raised the issue with
local staff at the Open Source Design forum: