Raster images use pixels, which is what Paint.net is, it is a raster program, as is photoshop.
Now if you take an image that is vector, like adobe illustrator, you will lose ZERO detail at any level of resizing.
In vector artwork, adobe illustrator for example, you take your pen tool, make an anchor point, make another point and drag, that is basically making an equation for that line you just made, inbetween those two points. With that equation you will lose nothing, no matter how you resize it.
Take a vector program, drag in a nice sized circle from paint.net, anti aliased of course, and then make one in the vector program, and zoom in and look at the difference.
I have a really good example of this, but it is saved as a PSD, I will save it as a png and show you the difference, was resizing a logo for a community service project, and I used a rastered square while I used vector artwork for the main part of the logo.