how do i delete pictures stored in the cloud

If you want to discontinue using iCloud Photo Library and delete all of the photos and videos stored there, see the following from Get help with your iCloud Photo Library - Apple Support

How do I remove all iCloud Photo Library content from iCloud and my devices?

To turn off your iCloud Photo Library, follow these steps:

• On your iOS device, go to Settings > iCloud > Storage > Manage Storage > iCloud Photo Library, then select Disable and Delete.
• On your Mac, go to Apple menu > System Preferences > iCloud. Click Manage, select Photo Library, then select Disable and delete.

Photos and videos will be stored in your account for 30 days. To download your photos and videos on your iOS device, go to Settings > iCloud > Photos and select Download and Keep Originals. On your Mac, open Photos, choose Photos > Preferences, then select Download Originals to this Mac. You can also select the photos and videos that you want to download from iCloud.com.