When it comes to an original piece, most artists will use portions of references and put them together to make a final piece. Using tools like Photoshop, GIMP, or Pixelmator to get proportion, position, and color palette of your painting will help a lot. Many times I will create everything in an image editor, then use that as my reference. What you can also do is to take reference photo's off the web that have the textures you are looking to achieve in your original piece, and have them on hand to use in your painting. One thing that I practice a lot is to take sample pictures off the web and I will paint a small section of the very textured parts of the painting a few times. Most times I just do it monotone so I can get the technique down. Once you practice that texture a few times, adding it to an original painting becomes much easier.