I run into this problem too. In my case I finally figured out that it’s an image size problem, as the other commenter said, though the error doesn’t message that, it just fails without explanation. What I would like is an image resizer within the app so I could just shrink it down, and generally some way to know before failure what I’m supposed to do. Some of that could be solved within Boost, but I understand not all of it.

Another person said that instances will encourage uploading to imgur or catbox - but catbox files don’t get previews on Boost, so that’s no good. At least for me anyway - but that’s kind of a separate topic.