How do I log in to my I cloud account to delete photos?

Use a web browser and sign in with your Apple ID and password at this link www.icloud.com

Tap a single photo and click on delete button , a pop up window appears again click on delete button .

You have to follow the same procedure to delete other photos as selecting of thousands of photos is not possible for that you have to set up iCloud photo library see this article https://support.apple.com/en-in/HT204264

And to select all photos use command and A keys to select some photos , use trackpad and move three fingers from top left corner to bottom right corner and click on delete button .

Again a pop up window appears .Click on delete , this will delete the photo from www.icloud.com as well your iPhone and moved to recently deleted folder and kept for 30 days , after the time limits the photos will be permanently erased from the iCloud server .See this article https://support.apple.com/en-in/guide/photos/delete-photos-or-recover-deleted-ones-pht3bfab2f96/mac