Google Recommends You Verify Your Hosted Cloud Provider Host In Search Console

"If you're using a cloud provider to host images / videos / other content, you can and should verify the host in Search Console, so that you're aware of potential issues that affect Google's crawling & indexing, & Safe Browsing. Use a DNS CNAME to the bucket, then verify with DNS. Using your own hostname (something like content.your-site.com) means you can verify it in Google Search Console to get crawl errors and malware alerts."

"You can verify using DNS verification... or just verifying your main domain. To do this, set up a CNAME entry for your domain name and point at your cloud provider's bucket, eg "content.your-site.com" uses a CNAME for "your-bucket.clodstorage.com" (or "buckets.clodstorage.com"). Also, you will have to update all links in your site (ugh, I know)."

"You need to update all the links within your site so that users only find your content with your new hostname. For bigger sites, this is a hassle, I know. Search & replace, then double-check by crawling the main sections of your site (all templates, all important URLs)."

Verify custom hostnames used for cloud-hosted images, videos, and other assets in Google Search Console by creating a DNS CNAME that points the hostname to the cloud storage bucket and then completing DNS verification. Verifying a custom hostname allows access to crawl error reports, malware alerts, and Safe Browsing notifications for those resources. Implementing a CNAME requires updating all internal links to use the new hostname and crawling key site sections to confirm template and URL coverage. Image Search may fluctuate because images are often recrawled more slowly than web pages, so traffic to images can change after migration.

Read at Search Engine Roundtable