Can I delete Amazon snapshots that were created by CreateImage?

Only if the snapshot is not used by an AMI.

When you create an image of an instance, AWS will automatically create a snapshot of it first. This snapshot is needed by the AMI, so you cannot delete it. Fortunately Amazon will warn you when the snapshot is needed by an image, so when in doubt, just try to delete it in the webinterface. You will then get an error message that the snapshot is currently in use.