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Art-to-Part is the term for taking an visual representation of something and turning it into a tangible part.
The first step is to vectorize the picture. You need a good picture to start with... there has to be enough detail. Often times, a friend will show me a picture of something they would like me to make. Unfortunately, when I zoom in on the picture I'll find that there is no detail. It also helps if the object in the photo is in the same plane as the lens of the camera (co-planar). For example, a photograph of a carving on a cyclinder will be tough duplicate in CAD because of perspective. 
You can vectorize an image by using short straight lines or curves. Why would you want to use short straight lines? Sometimes when "porting" (going between software packages) the polylines and polyarcs don't translate well. Mikhail Kalashnikov once said, "Perfect is the enemy of good enough." There is a lot of wisdom in that saying. Engineers who try to make something perfect, often end up with something that isn't even good enough. Small line segments don't show up when routing an object. They are faster for the CAM software to process too. Often times, they are just simply good enough.
Vectorizing basically means that you are tracing the image. One thing that is really important is that you keep your drawing "clean" and by this I mean that you use snap functions to make sure that all lines that should be connected are connected. This is also sometimes called closed-loop vectors or valid math data. You copy the outline and the details as much as you need for what you are making.  There are some programs like Power Trace in CorelDraw or Vector Fit in VCarve Pro that will trace an image for you, but don't be surprised if you can do a better job by hand. 
After you've got a good design, the next step is to import the drawing into a CAM software. This will let you turn the vector art into a real part. Most of the CAM software programs I use really love AutoCAD Release 12 version of vectors. This is a very stable format to go among RhinoCAD, QuickCAD, CorelDRAW, Cut2d, and VCarve Pro. Even LazyCAM likes the format AutoCAD R12 .  
A good CAM program will also let you simulate what the part will look like when it is finished and even tell you about how long it will take. This prevents a lot of wasted wood. It will also show you how different cutters will perform when cutting pockets and profiles. If a cutter is too big to make all the details you need, you'll see it and decide to switch to a smaller cutter. Once you are satisfied, you'll use the CAM software to create a toolpath file (often call a G-Code file). In it will be all the information your router will need to make the part.   
As amazing as CNC technology is... when you do design something, you have to keep in mind the limitations of the tools you will be using to make the part. For example, all CNC routers will radius the internal corners of all objects they cut by the radius of the endmill or router bit. The smaller the diameter of the endmill you are using, the smaller the radius will be... but it will still be there. A cnc router will cut all external corners perfectly square. It is a problem if you are cutting inlays, you need to make the external corners match the internal ones. Somethings that CNC just isn't good for are: cutting deep through materials using small diameter endmills. Small endmills can cut tiny details, but they are usually very limited in how deep they can cut before breaking. Special long reaching endmills are expensive and they break easily. With time, you'll learn which projects you need to modify in order to make with CNC. Some of my friends are often surprised to find out that I still cut out the items I make with a scrollsaw. It is all about using the right tool for the job. With the right blade, I can turn sharp internal corners and actually cut something out faster and better than my CNC.
Many times, my friends will see the amazing work that the CNC can do and they will think this wonderful machine will replace woodcarving by hand. Not a chance... Sure, some types of carving can be best done on a CNC. But those designs that have sharp corners are a real problem. Rastering over the surface, a ball nose endmill can't reach into all the sharp corners that make many designs so beautiful. To try to achieve this, smaller and smaller endmills are needed... which leads to longer and longer machine times. It could take hours to accomplish on CNC what it might take me a few minutes to do with a chisel.
Woodcarving by hand is art. I show myself, my talents and flaws, in every thing I carve. Every piece is different - even when making many of the same design. A beautiful thing happens in the synergy between the two, man and machine. The machine roughs out the design for me (that's the part I hate). I supply the artistic, the fine detais (the part the machine just can't do). My CNC and I, we work well together.