Adding Coverarts to same-name-mp3file

[ohrenkino]

You have to use an action.
See the FAQs on how to use/create actions:

Actions and batch-operations

How to create a new action? First, create a new action group via Actions > [Actions] Actions or Alt + 6. Then, click on the button [New] New and give your action group an expressive name, e.g., "All tag fields Upper Case". This action group can be applied to the selected files after you have added one or more actions to it. Now, you can create a new action in your action group via the button [New] New and choose the desired action type from the list. At the moment the following action types …

Also see there an action to import covers:

Import Cover from File – Mp3tag Documentation