I just tried to upload a couple of screenshots and on clicking “Upload” it failed saying it was unable to determine image size, and that “maybe” the image file is corrupted. The images are fine - I just uploaded them to the SAFE Network discourse forum here - so I’m not sure what the problem is.
It isn’t important in this instance, but it would help to be able to upload screenshots in future so I’m flagging it up now for information. Is it perhaps because your version of discourse doesn’t understand PNG files?
My first guess would be upload dir permissions, but could be anything. We just updated Discourse to the latest version last Friday, so I don’t think it’s a missing-feature problem.