When you delete snapshots and images from the Control Center, they are only marked as deleted. They are not yet deleted from the I-Fabric. Until you delete them from the I-Fabric, you will not be able to create snapshots and images with the same names as the ones marked as deleted.
To purge snapshots and images from the I-Fabric, issue the image -lR command from the control plane server to view a list of the images marked as deleted. Then issue the image -d command to delete them from the I-Fabric. See the image command man pages for details. (4856854)