In camtasia, go to effects --> options, and then the watermark tab.
Choose the path to the image you want to broadcast.
Check "overlay", uncheck "keep aspect ratio when scaling", and slide the "image scale" slider to 100%. This will fill the viewers' windows with the image you've chosen. In NSVcap make sure to uncheck "capture audio" unless you want to broadcast your PC's or you mic's sound.
You'll still, however, have to actually capture part of your screen and broadcast it with camtasia for the overlayed watermark to show up in your stream. If you really want your dead air screen up all the time, that means dedicating this somewhat CPU intensive program... all the time. The other method I mentioned might be better, as you're really only using upload bandwidth on a constant basis, as opposed to your processor and memory.