Filter for files that do not have an embedded cover:
%_covers% MISSING
Create an action of the type "Import cover from file"
Format string: %_filename%.jpg
This will probably lead to a number of error messages for all the files where there is no cover file with the specified filename either because there is no file at all or the filenames do not match.
Press F3 twice to refresh the filter.
Create another action of the type "Import cover from file, this time with
Format string: d:\covers\%_filename%.jpg
and see how many successful imports you get now.