I'm going to show you how I made the this background: Background - Lightning using Paint.NET.

First of all, open Paint.NET. Then, click the drop down window "Image", then click "Canvas Size". Now change the width to, we'll do "1200", then change the height to "1200" (Just to make sure our image is big enough for background purposes, you can always trim it down as needed).

Your canvas should now look like this: Canvas

Now we're going to do the drop down menu "Effects" and then choose the effect "clouds". This image is simple, so just click "ok" (The default cloud settings work for this).

Your canvas should now look roughly like this (I say roughly because the pattern of the clouds are random): Clouds

Now simply click "Effects" again, then click "clouds". Now, this time we're going to change the blend mode to "negation". Once you have done that, click "ok".

Your canvas should now look roughly like this (I say roughly because, just like before, you guessed it, the pattern of the clouds are random): Finished Image

Note: The images are rather big, I may make them smaller at some point, but for the moment that's why I'm using links.