Idea

You can actually already do this offline! Simply go to your Marble Blast Platinum folder (it's in AppData/Roaming, you can look up how to get to that folder if you don't know how) then go to platinum/data/custom_marbles/custom, and edit one of the skins there, or make your own (it MUST be 256x256, 512x512, or 256x512!) and save it as "<ANYTHING>.marble.jpg". When you open your game, go to the offline level select and open the marble selector. Click "Cat" until you reach your Custom folder, then pick which skin you want to use.

Unfortunately, multiplayer/online does not support this. Sorry about that.

Simply go to your Marble Blast Platinum folder

If you open the launcher for MBP, press Marble Blast Platinum, and then click on "Open Game Dir", it will open the AppData/Roaming directory where MBP is stored (or Home/Applications for Mac OS X).

To reiterate what Threefolder said, there is no plan to support custom marble skins on either online play or in Multiplayer.

There is a way around this (that BPX told me about) but it involves "deleting" the original skins.

Just take a marble skin you want and replace any of the original skins with your desired marble. Make sure your rename the file.
Afterwards, start the game up with the .exe file, not the launcher. You're marble should be in the same spot as the old original one is.

If you want the original marble skin back, run the launcher.

Rozi wrote: Make sure your rename the file.

rename it to be the same as the name of the marble skin you deleted.