If you are using the Free Plan, photos clipped to Today's Cover are uploaded and stored in the cloud, but other photos and videos recorded on the timeline are not uploaded to the cloud.
Therefore, please note that when you unclip a photo from Today's Cover, if the corresponding data does not exist in your device's photo library (gallery), it will no longer be displayed on the timeline.
For more details about the differences between the Free Plan and the Premium Plan regarding photos and videos, please refer to this help page.