Great, thank you :-)
I could not find one of the directories you mentioned, only
and both were empty
BUT because of that I realised that they weren't Poweramp-cached covers, but my own cover.jpg files which are located in every albums directory.
AND Your hint with .nomedia solved my problem by placing one single . nomedia file in the main/top sound-directory where my mp3s are.
Thank you very much :-)