On occasion when we see this issue it's typically a result of images that exceed a certain file size. If you are experiencing this issue, it's likely that an image larger than 1280x720 has been uploaded.
A temporary workaround would be to reupload those images but reduced in size.
This is a known issue and one that only recently came up as a result of an update. We are working on a fix for it and apologize for the inconvenience.