Board Thread:Support Requests - Getting Started/@comment-34981175-20190130150728/@comment-34858178-20190130155544

By default, MediaWiki has a maximum image dimension in pixels to allow before thumbnail generation breaks and the file can't be uploaded. I believe the current maximum dimensions are 3500×3500. The full sized image on that link is 7720 X 4409. I'd recommend resizing the image to 25%, which should still upload a fairly generously large image while getting under that thumbnail limit.