Photo Library on Mac -- how to store it in iCloud?

Your best bet is to get an external drive, either HDD or SSD (if budget permits) and backup your entire user data to it with Time Machine. Or if you want an archive of your image files get an eternal SSD of a size that will hold all of your photo files and export all of them out of your Photos library, using the albums as folders, to the SSD. I have my 58,000 image and video files archived to a 1 TB bare SSD in this configuration.

I download my photos from mh iPhone to a folder on the Desktop once a week and add them to the external SSD and import into my Photos library.

I also have an external drive for Time Machine backing up both by boot drive and eternal SSD.

You should be aware that the iCloud Library is not an off computer storage facility. It's a syncing service so that all of your devices that use the same Apple ID and have enabled iCloud Library will have the exact same photos in each library.

However, any changes made to the library, i.e. additions, edits and deletions (even accidental deletions) will be reflected in the iCloud Liberty after 30 days.

So it's not an archival storage situation. That can only be achieved by copying your library to a second external drive and updating it every time your working library is updated.

Move the library to an external disk formatted Mac OS X Extended, Journaled or apfs.

I think we need to be clear here. There is iCloud Library, which works like OT says, then there is iCloud Drive which acts like an external drive.

However, Apple specifically says that you cannot use iCloud Drive for your Photos Library, so that option is out.

There ought to be some sort of Cloud based storage system where you keep your archive of photos and can upload new ones...I will keep researching but do you know of anything like that?

The very obvious ones are the so-called photo sharing sites like Flickr and Smugmug. Upload your images to one of those. Set the permissions for as much privacy as you want, and you have a: a good back up of your photos and b: your photos available on any computer in the world that you might sit down to.

Anther alternative is Adobe's version of the Cloud: Lightroom CC. Unlike the Apple version Adobe do store your images for you in the Cloud, and so take responsibility for your back up. (It doesn't preclude you also having a local back up). Downside: cost. Lightroom CC is a subscription app, but it is a very good one and the range of features is very strong. It's much more capable than Photos, but then you do pay for it. If you do cancel your sub you have an entire year to download your material from their servers, so it's not draconian. Worth checking out.

Why trust some random people with your images?