Does everyone in your multiplayer game need the same graphics? Like the Graphic Improvement Project or Scytth's icons? 
Will the game still be able to run?
Will the game still be able to run?-
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Players dont have to have the same graphics to be able to play together online. If you are unsure on what can and can't be done, just look at the checksum. If it stays the same after changes then you should be ok in MP games.