[iMore.com]

Are you ready to go all-in with iCloud Drive? You can move files and folders from Dropbox, Google Drive, or OneDrive right now!

Full story from the iMore Blog...

[Sherry_B]

If I delete a photo from my phone, does it also delete it from iCloud Drive, or does it keep the photo in iCloud Drive but delete it only in iCloud?

[Tartarus]

If I delete a photo from my phone, does it also delete it from iCloud Drive, or does it keep the photo in iCloud Drive but delete it only in iCloud?

It only deletes it from the photos app on all your iCloud Photo Library enabled devices.
If you had some photos explicitly saved to
iCloud Drive, it will not be deleted, unless you specifically delete it from iCloud Drive.

Here is some extra information:
https://m.imore.com/icloud-photo-library

[Ledsteplin]

It only deletes it from the photos app on all your iCloud Photo Library enabled devices.

Doesn't deleting photos from the device also delete them from iCloud back up or iCloud Photo Library?

[Tartarus]

It deletes it from iCloud Photo Library. When iCloud Photo Library is enabled, your photos app on any device essentially becomes iCloud Photo Library, linked to each other through your iCloud account.

iCloud backup doesn’t backup your photos if iCloud Photo Library is enabled. It leaves it alone.

[Ledsteplin]

It deletes it from iCloud Photo Library. When iCloud Photo Library is enabled, your photos app on any device essentially becomes iCloud Photo Library, linked to each other through your iCloud account.

iCloud backup doesn’t backup your photos if iCloud Photo Library is enabled. It leaves it alone.

Not everyone uses iCloud Photo Library. I don't.

[Tartarus]

Not everyone uses iCloud Photo Library. I don't.

I figured that I pretty much covered that in my first answer.

To summarize:
If you have saved photos to iCloud Drive, it will stay there if you delete it from the photos app.
If you have iCloud Photo Library enabled, deleting a photo from photos app will delete it from all iCloud Photo Library enabled devices and from iCloud.com/#photos, but not from iCloud Drive. You will have to delete any files explicitly from iCloud Drive.
Enabling iCloud Photo Library replaces the conventional way of photos backup in iCloud.