Hey Andres! Very cool sculpts! For the "halloween" one I suggeest to put character in some environment, or the simple 'halloweeny' gradient or theme and it will be great poster).
And about textures... This is all depends from your style. You can go without textures and just do handpaint like oldschool style, or you can sculpt a facture of surfaces and then do same handpaint, or if you do decide to do texturing for realtime PBR, then check some tutorials with character texturing. Even if you don't have any software like quixel suite, substance painter or 3d coat, the principles is the same everywhere... Good luck!
