Can I retrieve a picture that was deleted from my iCloud ?

You can if it hasn't been 30 days since they were deleted OR if they were not permanently deleted from iCloud:

https://support.apple.com/guide/icloud/recover-deleted-files-mmae56ea1ca5/icloud

You may also have a shot at retrieving it from backups.

Marashalick Said:

"Can I retrieve a picture that was deleted from my iCloud ?"

-------

Downloaded Photos:

Have a device that these were downloaded to? If so, they would be recoverable on that device, even if they were uploaded to iCloud. That is because they would have been already downloaded on it.

Avoid Syncing:

Have a device that is associated to this Apple Account, but is not connected to the internet? If so, disable your network (by powering off your router), open the Photos app, and then take a screenshot of each photo.