@timcliff I just tested this by using "https://cdn.steemitimages.com" and all is good to go. Make sure you do not include the trailing '/' on the upload_image directive in the config.
"upload_image": "https://cdn.steemitimages.com"
(The trailing slash NEEDS to be on the img_proxy_prefix)
"img_proxy_prefix": "https://cdn.steemitimages.com/"