At what point Zotify attaches original album covers to converted .mp3s metadata?

[aari38]

EDIT: I was wrong, Zotify imports album covers to ogg files, it's just Windows explorer which doesn't read them I've managed to convert ogg files to mp3s using ffmpeg keeping all the ID3v2 tags including album covers eventually.
If you need help converting using ffmpeg ask me in this thread.

I've found that original .ogg files downloaded by Zotify don't have album covers in metadata, however when Zotify is set to transcode to .mp3, the album covers are attached. Can someone explain? I download .ogg files with Zotify and then batch transcode them to mp3s using ffmpeg, but I miss album covers.