Sorry hideously behind forum stuff this week due to real life getting in the way, tanks to Ian for looking into it and getting to the bottom of the FB issue! From an image embed point of view, I know Rick's issue has been sorted and it was due to image size.
For the images that you're posting
@skyscraper, after having a look with Ian it appears to be a Discord issue. Basically the image isn't found by the image proxy plugin. I'm guessing Discord's image hosting sits on a content distribution network and for some reason images are propagating across it. I was able to replicate it where your image loaded on my connection, then connecting to a VPN and trying to view it failed. The image proxy will be hitting one of these servers where the image doesn't exist, whereas you're not. Very bizarre and random to say the least!