This is more of a theoretical question. I was looking around on youtube and saw a video on texture painting and was wondering if I could make it more realistic. I don't want to use the Internet for this because I want to challenge myself and two I just started learning about blender and three my wifi like my imagination is obviously terrible.
In Node Editor Add two color textures (Image Texture nodes), set one as Color and the other as Non-Color Data. Connect the second one to the Height input of the Bump node (set the Strenght as you like). Plug it then to the Normal input of your Diffuse node.
BLENDER INTERNAL RENDER
You can totally do that! I am using a normal map here, along with a texture I already created. So you should already have a texture set up on your object, looking like this:
Select the next texture slot down, and click "New."
Open the image you want to use as the normal map.
Scroll down to the section titled "Influence," and disable the Color, and enable the Normal portions.
Depending on your UV mapping, you might want to switch to Generated instead of UV (if you where using UV, you also have to select the UV map you want to use). That's what I did here. Then, change to Material mode to see your texture with the normal map. Keep in mind that this does not add any geometry, it just changes how it calculates the lighting to make it look 3D. You can see the effect on the bottom portion of the 3D view.