Thanks to Sebastian the cover export and import worked fine for me with the $validate(%artist% - %title%,) to assure that for the file system operations only allowed characters would be used (: or ? will be replaced with nothing).
But if I have multiple covers like front & back I got exportfiles like
Survivor - Eye of the tiger.jpg
Survivor - Eye of the tiger(1).jpg
$validate(%artist% - %title%,).jpg works only for the first cover.
So I tried this for the second
$validate(%artist% - %title%(%dummy%),).jpg
but this doesn't work => do I have to mask the ( with /( and /) ...
Hope someone has good tip to get this work maybe with only one action .... 
Thx
UPDATE:
I made one action with several similiar actions and that worked fine
Action Import Cover
$validate(%artist% - %title%,).jpg
$validate(%artist% - %title%,)(1).jpg
$validate(%artist% - %title%,)(2).jpg
$validate(%artist% - %title%,)(3).jpg