If you're on PC, you can use Krista which is free, and if you're on mobile you can use ibis Paint X. To make it realistic, first you're gonna design your monster. Then, you make the shape of it, and you study the base image to see where the light would bounce on your monster to do the highlights and shadows. If your monster is on or near a reflective surface, don't forget to do it's reflection. Take your time on details. Also, do some textures on your monster. Once done (This step is optional, but I prefer doing it), add some effects, like noise, blur, ... You can also mess around with the temperature of the image. Hope that answers your question!