User blog comment:CitizenA/Paint.NET silhouette plugin/@comment-24.184.105.92-20130327060430/@comment-5906821-20130403144225

Like the other guy said, you need to have a uniform background color that's different from any color in your image you want to keep. Nothing the paint bucket tool can't fix, just paint the background bright green or something and re-apply the effect. It should look like this before/after the effect. http://imgur.com/a/wgj98

Also, when resizing your canvas to 800X600, you can choose a center anchor for the resizing, which will avoid you having to move the sprite yourself. Just make sure to re-put it at left when doubling the width.