There are a few ways to do this.
1: This way is the most complicated and confusing, and only works if you have DB Classic. Use TEXTURE BACKDROP filename$ I do not recommend this.
2: Set DB to render 3d in front of 2d, this may cause computers to run slow if their cards aren't powerful enough.
3: The easiest way is to make a plain and texture it, then place it in front or behind objects as you need, you can even retexture it to change the background.
At_the_Gates used method #3 quite effectively.
*Copyright At_the_Gates of course.
If the image doesn't work go here: http://hem.passagen.se/emeraldia/testpic03.jpg
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Crazy Donut Productions
Current Project: KillZone