A, looks real sexy! Can't really say the oil painting makes sense but who cares? :P I'm assuming you are using UDK decal actor? http://www.hourences.com/tutorials-ue3-decals/
You would need a multi-layered shader and a way to manipulate the position, rotation and scale of the decal's UVs. An example: https://forum.unity.com/threads/iphone-shader-simple-texture-with-decal-2-uv.30629/
@DeathstrokeFTW LVG is on point with the video. And where you place decals inside decal sheet does not matter as long as they don't overlap. And on the mesh you just place them where ever they make sense.
What version of UE4 you're using? Recently i've been having problems applying decals since i'm forced to use the blend mode as translucent, which makes the decal color barely noticeable.