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@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/"