Paint.net is good, but cannot easily draw Text with a white background?

The only way I know of right now would be to create the text in a new layer above, then create a new layer below the text in which you draw a filled white rectangle. Then you can merge both again.

You could also format the text appear with an outline color which is different from the text color, then you don't need a white background rectangle to read the text. Most decent graphics tools can do that, surely Paint.net too.