Apply the action "Import cover from file".
This imports the cover art specified by the absolute or relative filename into the tag of the file.
You can use placeholders like %artist% or %album%.
You can use wildcards like ? or *.
So it should be feasable to do ... having ...
music file: Queen - Bicycle (128kbit).m4a
image file: Queen - Bicycle (HQ).jpg
... apply action ...
"Import cover from file".
Format string for image filename: $regexp(%_filename%,'(.$','')
Import cover as: Front Cover
For image filename which is not exactly the same as the music filename, you may use a format string like ...
$regexp(%_filename%,'\W','?')'.jpg' ==> "Queen???Bicycle??128kbit?.jpg"... this replaces all characters, which are not letters or numbers or underline character, with the ? wildcard.
$regexp(%_filename%,'\W+','*')'.jpg' ==> "Queen*Bicycle*128kbit*.jpg"... this replaces all sequences of characters, which are not letters or numbers or underline character, with the * wildcard.
$regexp($regexp(%_filename%,'\(.*\)$','*'),'\W+','*')'.jpg' ==> "Queen*Bicycle*.jpg"... this replaces all sequences of characters, which are not letters or numbers or underline character, with the * wildcard, and replaces the trailing parenthetical expression with the * wildcard.
DD.20140503.1538.CEST