To pixelize naughty bits in GIMP at least, it's really easy.
Simply marquee select (or lasso select) parts you want to cover, then choose from the top menu FILTER, and then BLUR, and then choose "PIXELIZE..." It's really simple. Just make sure you have the preview checked and you can use all kinds of other controls to adjust the rate, size, intensity, etc.
For Clip Studio Paint, it's basically the same instructions. Choose your selection, click on "FILTER" up top, choose "EFFECT" and then "MOSAIC..."