Even the cover file has 0 kb size, mp3tag imports them.

Mp3tag imports 0 kb size cover files. This happens especially when working along with Album Art Downloader XUI & DisCoverArt.

Does this mean ...
... 0 kb is equal to 0 byte?
... Album Art Downloader XUI presents 0 byte images?

Using the dialog "Extended Tags..." it is not possible to import a zero byte dummy file of filetype jpg.
But a jpg or png file with at least 1 pixel will be imported.