In December of 08' I published a tutorial on creating your own Blackberry themes using the Plazmic Theme Builder software program. I identified the basic steps and layout of the software and gave more advanced users a quick introduction to using the program. For beginner's and many less advanced computer users that tutorial will not provide the necessary instructions and information to begin your theme building process.
The following tutorial is a more advanced look at the Plazmic Theme Build, if you have not read the previous article its suggested you do so before continuing any further.
Also, before continuing make sure you have all the necessary software and tools installed which were introduced in the previous article; most important of course is the Plazmic Theme Builder.
Upon program loadup you will be prompted to enter a desired name for your theme, device screen size, and type of home screen layout you would like. For the purpose of this tutorial simply enter "Tutorial" in the name entry field, choose 320*240 screen size, and "custom" from the type drop down menu. 
After Plazmic has loaded up and you have chosen the above options you will be presented with a screen similar to the one on the right.
Our first task is to design the layout of the device home screen. Of course this generic roadway picture for our background isn't very attractive so lets find something interesting! Find a picture (any picture) and resize the image to the same resolution you chose for your screen size (In this case 320*240); you may be required to crop the image if the size does not corrolate correctly to keep the image from looking distorted. After the image has been cropped and resized save it somewhere on your computer that is easily accessible, such as your desktop. Once you have your image resized and saved return to the Plazmic Theme Builder.
In the top right box, select the second button on the left and the options in the right hand box should change like so (Below Image).
Once the screen resembles the picture on the left you need to click inside the box next to Background; a file browse window will then pop up and you must then navigate to where you saved the image you resized for your background and select it.
Then voila! The background on the left side of the screen (inside the picture of the Blackberry phone) should update, and if everything was done correctly you should see your custom image!