Minecraft Pixelart is a process where one turns an image into a pixelated image and then builds that in minecraft.
You can build things just from your head, or you can copy of images that other people (or yourself) make. It takes a little practice to start making these in minecraft but once you get the hang of it its fine. :) In the pictures i have you can see that some have little squares on them. They symbolise the squares in minecraft. For each one you see you put a block in minecraft in the same pattern and colour. Some do not not, and so you will have to use you eyes to guess how many blocks need to go down. Hope that makes sense.
You can make simple pixelart and complicated ones. (I did not make the bowser one way to complicated) Often you will need to try a couple of time to get the right effect in minecraft.