is there any possibility to use album pictures without saving them in your files (e.g. id-tag) or in your album folders? i thought of a kind of "album art library" or something like this. can someone help me?

With default UI, No.

For another UI. Yes, Rename your cover to have the same name of the album and put it to somewhere else and tell albumart viewer to look for cover that have the same album name at the place you put all your cover together.