Using OPTION THREE or OPTION FOUR in the tutorial below will help show you how to delete specific system restore points.
System Protection Restore Points - Delete
If you created a Windows Backup, then OPTION THREE in the tutorial below will help show you how to delete a system image backup using it's "Manage Space" option.
Backup User and System Files - Delete Backups
If you only created a system image backup, then you could open the WindowsImageBackup
folder and "Continue", open the ComputerName-PC
folder, then delete the Backup year/month/day time
folder for the image backup that you wanted to delete. The blue NOTE box at the top of the tutorial below will help show you more about what to look for with this folder to help.
Backup Complete Computer - Create an Image Backup
Hope this helps,