I think your going to find this difficult with just SMB as that only gives you access by default to the home folder and neither the splash image or the two skins that are installed by default are in there. I would start with a finding a tutorial with the basics of working with the Linux terminal. The most basic functions are all you really need for this. If you are using a current version of Windows 10 you can open the command prompt (win key then type cmd and hit enter) and then type ssh osmc@[the.ip.address.of.your.pi] to get to the terminal. From there you could just plug in a thumb drive with a suitable image named correctly and type in df to find what it mounted as. To copy the splash you would type…
sudo cp /media/[name you found using df]/splash.png /usr/splash.png
You should be aware that is will be overwritten at some point by an update and will have to be redone.
As for changing the skin that would depend on what skin you were planning on changing.