Blender has a built-in feature called Texture Paint that allows you to either paint directly on your model, or on the UV unwrapped (flat pack) version of the model. Work on one and the other automatically updates! However, photo-editing tools such as GIMP provide more flexibility when it comes to texture creations and have an abundance of brush strokes. Your flat pack version of your model can be important onto GIMP, worked on, and then reintroduced into Blender.