My advice would be to avoid generating photo-realistic source material, it's much harder to retouch, too many gradients. I try to generate stuff that's close to my normal painting style, makes it far more easy to put together. It helps to read up on traditional painting techniques and materials; "a gouache painting with ink outlines" works well for me. Gouache was commonly used before the advent of acrylics by commercial (magazine) illustrators, and lends itself to a style that has instead of gradients many separate flat colour areas that give the illusion of gradients. Also I must say that these pictures are cobbled together from many, many different generated pics, with a good deal of overpainting, I'd never get results like that from the get go (I've amassed quite a library of components by now 🙂 )