If I'm not mistaken, I believe you're using Cerebrata Azure Management Cmdlets
as Windows Azure PowerShell Cmdlets
do not have a Rename-Blob
Cmdlet (Cerebrata has it).
To use Rename-Blob cmdlet, please try the following:
Rename-Blob -BlobUrl "https://ttseast.blob.core.windows.net/images/Add.png" -NewName "https://ttseast.blob.core.windows.net/images/add.png" -AccountName "ttseast" -AccountKey "<your accountkey>"