You could use the screen effect in photoshop... idk about Paint.net Basically it will remove anything that is black in the layer, so if you want black to be transparent just applay this to the layer with clouds or the finnished effect.
I used this tutorial to make a flame like text, only mine needed too look more like a cloud of electricity or static. I then made a background in photoshop using some basic meathods such as the noise filter for stars, adjusting contrast and drop off, and later adding a hazy nebula effect using clouds and clouds negative. There are two cloud layers, and they fade of at the edges of the screen for effect. This is now my splash page for http://www.zennovations.net