I think everyone has prety much said it all, but let me try to sum it up as simple as I can based on a single brick as you mentioned.
You had the basic form, its looks like nothink at this stage, so you need to add several eliments to get it looking real good for close up shots.
First thing is you do have to have it UV unwraped. There are two methods that one would use. The first is unwrap manualy. You choose where you wish the semas are to split the brick into a 2d, flat packed map. This method is best for if texturing in a 2D application like Photoshop. This kind of Uv unwraping looks clean in the way its laid out, and enables you a visual aid to what part of the map your painting on in 2D. Most use this method regardless, as it opends up the model to be textured by anyone with any painting application.
The next is a Auto unwraping method. Zbrush has these, often AUV, and GUV tiles are used. This makes better use of the full UV space, but can not be painted on in any 2D application, as the Uvs apear somwhat scattered, but usable by Zbrush via polypainting. The other advanrtage of using this method is that a single 4k map will hold much better detail as the Uvs are distributed better than most peoples manual uv mapping.
Once you have this done, your next need to add a colour map. Reason why your do this next is because you will want the bumps and details to match the colour texture details. For you, using a real picture of a high res brick would be the fastest way. You can use the brick picture for both the colour infomation, and for the displacment infomation.
First your either use a 2d program like photoshop or use Zbrush Zproject. For pure Photoshop you have make a UV template, a wire version of your Uvs. Using UV mapper classic can do this with ease. To keep it more simple you can use a plugin called Zapplink which allows Zbrush to work with Photoshop.
To keep things even more simple, your just use Zproject in Zbrush. When producing the zproject cast of colour infomation, you will need the mesh to be high resoloution to hold the high detail texture info, just as mentioned above.
You may want to retail the initial shape, so your store a morph target before doing any sub deviding of your mesh. Also you may want to keep the mesh from smoothing duing sub deviding, so you turn smooth off.
Now you can start to use Zproject to get that high res colour onto your model. Next, your need to get some high detail into the brick. Again there are many methods. First way would be to convert your texture map into a alpha, then displace with that alpha, all done in Zbrush.
When done, your have to go back to sub d level 1, switch the morph to get back the original brick shape, as all this sculpting on higher levels will alter the first level. When thats done your generate a 16 bit displacment map.
Once that is done, export your brick as a OBJ, if you havent already done so, and save out that displacment map which has to first be sellected from the alpha pallet, and fliped on the vertical via the alpha menu.
Delete and store your morph target again, should you wish to do another such map later, then go right back up to the highest level again. Make shure the RGB at the top of the interfacc is set to 100%, make a blank texture 4096 x 4096 from the texture menu, then in the texture menu to the right of the UI click colour to texture button. I dont have access to Zbrush for the exact name, its abreviated. Then export your texture after flipping it on the vertical.
Now you have a brick obj, colour, and displacment map. Take these 3 into your rendering application, set the lighting up and scene, add a shader to that brick so you can setup things like how the light effects the surface like diffuse amount. Set up things like the specularity level, for a brick quite simple.
Apply the colour and displacment map, render your scene. This is rather simplified.
Now do the same for the human head, but with these steps.
1: Store morph target.
2: Sub devide.
3: sculpt the head with basic deformations
4: make a new layer and add high end sculpting details, either by hand or wait and use the colour texture map as a alpha.
5: hand paint via polypainting, or combo of zproject.
6: complete step 4 if havent done so.
7: go down two sub d levels from the highest and generate a displacment map. This will be used as a bump map. Or make a normal map which il cover in a min. Normal map not done at this stage.
8: turn off the high res detail layer sculpt, go back to sub d level 1, switch morph target, and make a displacment map. save your project as a Ztl, then delete all lower levels so you only have a few higher ones left.
At highest sub d level turn on your high res layer. Go to sub d 1 and use Zmapper to make a normal map, a whole chapter can be made on this alone.
If your mesh is said to be too high res by Zmapper, use reconstruct mesh in the geometry tab to the right of the UI, this will make another lower level.
Now make shure both the normal and displacment maps are flipped on the vertical, there is a option in zmapper to flip the normal map when generated.
Now export all map, you havent done for so far.
In your render applciation the fun starts. You must make a skin shader with the right settings for how the light penitrates the skin, use Subsurface scattering for this. Your have to get this just right. Normaly a map gets made to control this. This would be a grey scale map. Darker shades alow less SSS, while light shades alow more. So your paint a map in Zbrush based of this. Lips, ears will have more lighter values, while a darker shade fo grey for the rest. Also maps can be used to control the specular, reflection, difuse.
Now you gota spend the time getting the perfect ballance between the correct lighting, SSS, the whole shader with every component doing its job at the right ballance, this is a art in itself.
Then if you have the skill, your have a realistic looking skin.
This is a quick run through, its not a quick process by any shot. The more time you invest, the better it will look. Lighting and bump detail is essencial to realistic skin, without these right, no matter of SSS and skin shaders will look great.
