I had already set img_proxy_prefix to "https://steemitimages.com/" in order to get all the images from existing posts to display properly, so this part was done.
I just tried setting "upload_image": "https://steemitimages.com", and it still gives this error when I try to upload an image through the site:
Im guessing something recently changed. I am having issues in my dev env now also. I'll update if I find the fix.
@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_imagedirective 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/"