For [Stream_General] we can query "Cover" (and its respective friends, like "Cover_Data"), which works fine as intended.
However, when inspecting a container (i.e. MKV) which comes with more than 1 audio stream (i.e. 3: one for each language), each of those may be tagged with IDv2 and as such each audio stream can have its own picture.
I haven't tested how MI is currently handling this (I hope it at least brings me one of all covers), but favor to have the chance to get all pictures. Hence all "Cover*" indices should also exist in "Stream_Audio".
For maintaining backwards compatibility the indices in "Stream_General" can still co-exist and would act like they already did. If MI inspects an MP3 with IDv2 tags, one should always get the picture via "Stream_Audio.Cover_Data" (and for backwards compatibility reasons "Stream_General.Cover_Data" should return the same).