Is this the filename or the tag field TITLE?
Is this what you use as synonym for the picture file that should become the displayer cover? I ask because there is also a tag field called LABEL ...
The "HowTo" section in this forum tells you how to create an action:
Something similar can be found in the help:
In the help you also find a description of the action to import a cover (picture) from an external file.
You fill find that you have to either enter a fixed filename or one that is created dynamically from tag field variables.
Depending on whether you quoted the filename or the tag field TITLE when you mentioned
you enter as
Format string: %title%.jpg
or
Format string: %_filename%.jpg
Try it with a test file.