I don't think this is related to the wiki. Please see thread 429893 on your test forum where I posted the same image in a regular post, and the same issue occurs. The issue is related to XenForo's image proxy. It seems to be caused by a 301 redirect because the requested image URL is actually stored in a different location. In order to prevent a major security issue, image proxies should not follow redirects. To avoid this, users posting IMG links should use the canonical location for images so that no redirects are necessary.