When your new iCloud account saw those pictures on your phone, it copied them to iCloud Photos. It does what's necessary to keep your iCloud Library exactly the same as your phone's Library, and it doesn't delete pictures without you telling it to. Pictures on the phone or at iCloud don't have a limited lifespan and just die off after. They last as long as they're paid for.
As long as you're connected to this account, if you delete these pictures from your phone, then they will be deleted from iCloud Photos. It's entirely up to you which ones you delete. Sorting by date, or using the search field may help to narrow down the work.
Good luck!