SolidWorks Composer is used for a variety of purposes, and while shop instructions and end-user instructions are the most common applications, many people are making graphical assets for marketing purposes. You can create more realistic images if you take advantage of textures.
You can place a single texture on any geometry actor. To get started, simply select the actor(s) and in the Texture section of the property list, check "enable" As soon as you do, this, the Textures property section will expand to show you more choices.
The first thing you need to do is point to a texture file, which is nothing more than a graphic file in .bmp, .jpg, .tga or .rgb format. It's best to have the graphic as a square tile that is repeatable, unless it's going to be used as a single scaled-up graphic.
From there, the defaul settings for Transparency (off), Blending (off) and Mosiac (on) are typically appropriate, although you can play with them to see their effect. The default setting for Projection Mode is Planar, but depending on your geometry you might get more pleasing results from Cylindrical or Spherical.
Note that you cannot have more than one Texture at a time, and the Texture is for the entire actor. What if you want to put a texture on a partial area of an actor? A simple way to do that to place an Image actor over the area you want; another way for some more flexibility is to create a Primative (more shapes available).
Finally, remember you can change the Texture on the Ground collaboration actor. I usually use a custom graphic combined with some Grid and Transparancy setting tweaks when I want to show the Ground. Sometimes I'll use a specific texture for concrete floor or grass, depending on the scene I'm trying to depict.
By the way, if you are looking for textures, there are a number of places you can go on the Internet. The one I find useful is http://www.grsites.com/archive/textures, but remember you can make these from a digital picture as well!