I know you have only moved this code here ...

TagLib::ByteVector::find returns -1 on not found, and we add 1, so this is safe here. But maybe we should return QByteArray() instead?

The code also works if only the picture data is there without the name.
I could return an empty QByteArray if dataPosition == -1 and pictureData.size == 0.
Do you prefer if I do that separately?

Does Taglib signal an error when it fails to parse the file? Or is calling pictureList() always safe?

Not sure if this is an issue, but the old code only called tag()->pictureList() after !tag()->isEmpty() - is this safe without?
Dito for oggFile above.

Is it safe to call find on an empty ByteVector - most likely yes ...
Then return QByteArray on dataPosition == -1, so it is obvious the error case is handled, and keep the rest as is.

Nitpick - add an empty line in-between functions

Docs don't say, so I fed a musepack file into this codepath and it still ran without issues (no cover extracted of course). Good enough?

Yes, the list will be empty.