Q: May I upload only pictures that I choose?

When you are using iCloud Photo Library, it will be all or nothing.  The purpose of iCloud Photo Library is to keep your photo libraries in sync across all devices, so all photos in your library will upload to iCloud.

But you can have more than one Photos library - for example one library with all your favorites that will upload to iCloud and another larger library will all other photos you want to keep locally.  You can simply switch between libraries by clicking the library you want to open.

See this page:   https://help.apple.com/photos/mac/1.0/?lang=en#/pht6d60b524

Hi,

I am able to upload selected photos at iCloud.com.

Go to iCloud.com and sign in.

Then at the top of that window click Upload then navigate to the photo or photos (you can select multiple photos by holding down the Shift key) then click Choose bottom right corner of that window.

I just tried this myself and it worked.

That is a possibility, yes, Carolyn. But that requires that you are not using iCloud Photo Library on your devices.

You could as well drop the selected photos onto iCloud Drive.

If you upload manually, no later edits will sync to iCloud, and neither will your albums or metadata. You will continually have to upload any updated versions manually, because you cannot edit and tag the photos at www.icloud.com. This has to be done in the Photos.app on a Mac or iOS device.

The syncing power of iCloud Photo Library will be lost.

Hi ...

Thanks for the info!

I'd like to read more about this in Apple support articles. Can you post links please?

There are not many support documents on iCloud Photo Library.  Which have you already seen, Carolyn?

The syncing is best explained in the Photos Help: https://help.apple.com/photos/mac/1.0/?lang=en#/phtf5e48489c

With iCloud Photo Library, all the photos and video in your photo library are stored in iCloud, so you can access them from your Mac, iOS devices, and iCloud.com. You must turn on iCloud Photo Library on any computer or device that you want to access your library. Any new photos you add to Photos or take with an iOS device are automatically stored in iCloud. If you make edits, you see your changes right away on any device that has iCloud Photo Library turned on.

It is implicitly said: If you enable iCloud Photo Library, the complete library will sync, and without enabling, your library will be local only and not sync.

iCloud Photo Library FAQ - Apple Support

Get help with your iCloud Photo Library - Apple Support

http://www.apple.com/support/mac-apps/photos/

This will explain how libraries sync between macs:   Use Photos and iCloud Photo Library on multiple Mac computers - Apple Support

There is no documentation for Photos at www.icloud.com. Just the Help button after you sign in: Overview

Try the section 

Use the Photos app on iCloud.com

It is very limited what you can do in Photos at www.icloud.com.  You can browse and create albums, vut not search or edit in any way.  It it nearly unusable for a large library.

We need to be able to open the iCloud Photo Library on a computer fro editing, searching, adding metadata and making a backup.