These may be Artworks from Windows Media Player.

You can disable Scan file directories for Artwork under Tools > Options > Library if you don't want external Artwork picked up automatically.

I have a PC running Windows 8
Some of the music files in my MM have 2 additional copies of album art.
There is the art that I have downloaded from my hard drive - and it is usually 600x600 or thereabouts.
In addition, there are 2 small copies - 200x200 and 75x75.

I do not require these 2 small copies.
How do I go about deleting them.
Note that I have tried deleting them, but they tend to re-appear unexpectedly.

Is it perhaps related to putting album art in folders - which I used to do.
But I now put all art in the tag associated with each music file.