Hi,
I have - let's say - a folder with a hundred of files following the format:
Song One.mp3
Song One.jpg
Next Song.mp3
Next Song.jpg
So every jpg file covers a mp3 file of the same name.
Is it possible to add those jpg file to the according mp3 files without doing it file by file?
Hm, looks like it is the solution, but...
where do I enter that %_filename%.jpg? I do not find the according menu 
You have to use an action.
See the FAQs on how to use/create actions:
Also see there an action to import covers:
Waoh, perfect - thank you a lot, worked like a charm!