For the yoyos, remove the white/black background with the magic wand tool. Just use the magic wand, click on the grey area, and most of the grey area will be selected. Play around with the tolerance.
Download some plugins. Use plugins. Don’t treat paint.net like how you treat paint, Paint.Net has powerful render effects. Use them instead of drawing random lines.
(Effects.rar includes lots of good plugins for you to use, download it and extract it to “C:\Program Files\Paint.NET\Effects” if you left everything at default when installing)
Also in the attached, a picture I made with random effects. With the “Random Maze” effect, “Julia Fractal” and a rectangle with a “downwards diagonal lines” fill.