How transfer from iPhoto/Mac 10.6.8 to iCloud without exporting from iPhoto?

There is now way to do what you want if you're on 10.6.8, except by exporting. Also, I would note that iCloud is a sharing service, not a back up. Further, the iCloud Photo Library is not accessible to a browser upload.

Current versions simply open the library with Photos.app and sync that way.

Icloud photos syncs everything accross all your devices. So if you accidentally delete some photos on one device, they get deleted in the cloud and on all the other devices. When they are gone, they are gone. No way back unless you have a backup somewhere else - which EVERYONE should have.

DogwoodD wrote:

But your statement regarding iCloud as a sharing service and not a backup is puzzling when Apple clearly markets it as a backup.

I think, that is a misunderstanding. I have not seen a single document, where Apple is claiming that iCloud Photos is a backup. Apple says "Keep your photos safely stored in iCloud", but that is something completely different. For example, the Photos User Guide is clearly describing iCloud Photos as a syncing service:

https://support.apple.com/guide/photos/from-icloud-photos-phtf5e48489c/mac

With iCloud Photos, all the photos and videos in your photo library are stored in iCloud, so you can access them from your Mac, PC, iPhone, iPad, or Apple TV, and on iCloud.com. 

Any new photos and videos you add to Photos, or take with an iPhone or iPad, appear on all your devices that have iCloud Photos turned on. Your photos and albums are organized the same way on every device, and if you make edits or remove items, you see the changes on all your devices.

If you are using iCloud Photos as the central storage of your photos, you are working with the iCloud Photos and you need a backup, so you can recover photos that you accidentally delete. And worse, if you are using iCloud Photos on an iPhone or another iOS device, your photos will no longer be included in your iCloud backup of the iPhone or iPad. And if you are using iCloud Photos and optimise storage on a Mac, it will also make your Time Machine backups incomplete, because the optimised originals are not included in the Time Machine backup. iCloud Photos is very useful for syncing, but does not suffice as a backup, it is even making backing up the photos harder. Three Good Reasons for Using iCloud Photos Library and when not to use it - Apple Community

I used Time Machine on the old Mac, then moved the library to a new Mac. I opened the library package and imported the original files into the new Mac, which will upload eventually to iCloud.

Why not just open the library with Photos and let it do its thing? What you did there cost you all the work you'd done on the images and organisation in iPhoto.

As to back up v sharing. Have a read of this

https://support.apple.com/en-us/HT204264

It describes the iCloud Photo Library as

iCloud Photos automatically keeps every photo and video you take in iCloud, so you can access your library from any device, anytime you want. Any changes you make to your collection on one device change on your other devices too. Your photos and videos are organized into Years, Months, Days, and All Photos.And all of your Memories and People are updated everywhere. That way you can quickly find the moment, family member, or friend you're looking for.

No mention of back up there. What it does say further down...

When you turn on iCloud Photos, your photos and videos automatically upload to iCloud. They're not duplicated in your iCloud backup, so you should keep backup copies of your library. You can download your photos and videos from iCloud.com to your computer and store them as a separate library, transfer them to another computer with Image Capture or Photos, or store them on a separate drive.

Emphasis added.