I clicked the "download album to picasa" button in the web client. The album was downloaded to the client beautifully and it was shown as a downloaded web album. Then I clicked the "sync to web" button in the client. It asked something and uploaded all images in the album. Why did they need to be uploaded again?
The problem is here. In the web client, there were 2 album now in the same name, the original and the recently uploaded. At first I thought it was not a bug, you might need to have a fresh uploaded album at the first time to keep syncing in the future and these twin albums was different album and unique.
and this idea leads to the mistaken operation.
I right-clicked at the downloaded web album in the client and chose to delete the album with "delete the online copies" checked. Then both original and recently uploaded album were gradually deleted! and they were deleted successfully.
So I don't have any copies of this album left. It's a bug of Picasa and I reported to its forum yesterday.
but good luck of me that the album has only 25 images and google still cache some of them. :P