I won't recommend Firebase.

With Firebase you have to store images in Google Storage. And Google Storage is one of the costliest file storage system. So far the cheapest options I found are

1. Digitial Ocean(DO) Spaces 5$ per month Linode also provides storage

You can't have your root domain on DO Spaces or Linode storage. Only sub-domains or sub-folders and files.

1. BackBlaze.com is alot cheaper than S3 and Google Storage. BackBlaze is S3 API comptable.
2. Host your own file server like owncloud (better NextCloud) on DO droplet or Linode. OR http://hetzner.com (this is the cheapest, but I haven't used it)