I have had this problem off and on for some time. I create a post, and the image uploads. but when the post shows up in a community feed, the thumbnail is the broken image. If you click through to the post, the image can be viewed by clicking on the image link. It seems to be image related, as I have tried scaling down the image and posting it again. My current example of this is. https://lemmynsfw.com/post/32709809 and https://lemmynsfw.com/post/32710178.
For what it is worth, I am using the web client under firefox.
Any idea on what is happening, and/or how I can know ahead of time that an image will be a problem.
Thanks!