Changing movie artwork,

[Aloush]

Hey guys

So I have some movies and I am trying to change the artwork on them.
It isnt allowing me on some of them and I noticed that on the ones I can't add artwork they are mkv's, Can't you add artwork to mkvs? and if not do you know a way so I can do it.

Thanks

 

[Queso]

What media manager are you using?

 

[Aloush]

I am just using finder and a shelf background.