A handy feature in Paint.net is the blur effect. This is handy if wanting to add a screen shot for clarification on something, but you don't want some sensitive data to be shown. The example below uses the Unfocus blur. You can set the radius of the blur as well. One simply needs to use one of the selection tools to select the area you want blurred and then select Effects, followed by Blurs,and then select the blur you want.
Click for a larger version.