I do not really view iCloud as a good storage place for pictures (or much of anything). It is more a syncing service. It will, for example, store and sync your 1000 most recent photos (depending on how you have your devices set up), but there is no ability to, for example, just copy a directory or a drive of photos up to the iCloud.
There are many other (and better) options for cloud photo storage. Flickr just increased its free storage to 1 TB per account. Some do not like the new Flickr interface, but if you are just looking for a lot of cheap storage, that's a good option. There are many others, including the online storage offerings by Microsoft, Google and Amazon. Some use Dropbox for storage; it is a wonderful service, but more expensive than Flickr.
There are also backup-specific services, such as Backblaze, that focus just on providing backup storage, and so are cheaper and to some degree easier to use for pure backup than the Microsoft, Google and Amazon offerings. I don't know your needs, but looking at these services is probably a good place to start.
I would never use cloud storage as my *only* method of storage; but you might want to use it in addition to your external drive.
