Sooo the image is part of the material that I've made for them. In Shading you can add a "Image Texture " node that lets you either insert an image or it makes a blank one for you.
Then in UV mapping you can save the transparent lines of what your UV map looks like. I put this UV map into MediBand Pro (it's free), which is what I use for 2D art on my computer, and painted on that what I wanted to have appear on the model.
So the image I was painting on, and the image I had plugged into my shader with an image texture node is the exact same. it's a .psd so it can have layers and be changed at will.
For the base colours and to figure out where goes what I used the texture painting feature that actually let's you paint onto the model and it then appears on the image texture.
I hope this answers your question :D
if not: This is the video I watched and followed for this