I've been asked by a few CGTalkers to do a tutorial on texturing a painting. Here's a brief introduction to some of the things I do to create an impasto look.
First, I used a few textured brushes that come with the default Photoshop library. It doesn't matter exactly which ones, because I don't really keep track. I just grab one and start painting after I'm comfortable with the value setup. The picture below illustrates my point, it's very textured as it stands. However, I plan to experiment and take it further.
I create a copy of the original layer and decide to run the Brushstrok/Accented Edges on it.
Now, filters are generally taboo, but if treated right some very cool things can happen. For instance, the above piece looks like typical filter-rama job. The trick, however, is to start erasing the copied layer at various opacities using various brushes. For this particular piece I mostly used the airbrush. You have to put forth the effort to search for things that work and remove the things that don't. Just experiment, have fun and when your satisfied merge it down to the original. Below, is my result.
Next, I grabbed a rock texture I found on Google. I pasted it into a Photoshop file, converted it to grayscale and saved it under whatever name you desire. Then, I went back to my Balrog painting, made another copy, scrolled down the Filter list to Texture, Texturizer, then Load Texture. Upon which, I loaded the rock texture, fooled with the settings and clicked "okay". Below is what I got.
As you can see, it's overkill. So, again, I begin erasing and having fun, until I arrive at something I like. Again I merge down.
Now, I try something different, I create a new blank layer, select a rake-like brush (looks like a series of dots) and under layer effects I add a small bevel to my stroke. Then, I start shoveling impasto into the opaque areas of the painting. I create three more layers and do the same on each of them. This way the strokes are separate thus creating a layered look. I even grab a grungy brush and erase out of the beveled stroke creating a pot-hole effect. Afterwards, I merge all the beveled strokes into one layer and, for the hell of it, I run Texturizer on it. Below, is what I got.
Here's a comparison. Some things work...some don't, but it gives you an idea of what can be done.