whoever owns a dsi i strongly recommend Art Academy from DSiware.
this is not an advertisement, I was quite surprised! I own DSi XL and it is super handy on them larger screens! many of our concept artists got jealous

it is a very nice painting tool on the go. and also very useful for someone who wants to improve their art skills or learn how to paint in the real world.
it is not painter, or photoshop, but it has very nice and pretty "realistic" brushes and pencils, it mixes colours realistically, (i.e. they will mud if you do too much mixing or use black or white to change your tone etc.) and it can give nice results i dare say even for painting low res textures since it can save on your SD card or even upload on facebook through DSi features and it only costs $8...
they have made two semesters as they call them i got the second one. i suppose what changes between them is the lessons and not the painting tools.
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sorry.. i can't delete it..
let me show you what i made the first hours i got my hands on it. one is a lesson i took to see how it works (nice lessons btw) and the other is a quick sketch.
a few things i loved and some things that could be better.
you can take a pic with your ds , show it on the top screen, then easily look and sketch\paint anything from anywhere.
something that i really liked! there is no undo when painting but only when you mix colours. so it allows the mistake to become part of your artwork.
of course you can cover with paint, or use an eraser when you use the pencil.
colours and pencils look quite realistic and the paint brushes change feel every time you click on them. it has 2H HB and 2B pencils and has straight and flat mode while sketching, for line art and shading. colours mix while you are painting (could be improved but works nicely)
you can mix your colours in a decent palette almost naturally. (would be awesome if they had painter's mixing area)
i would love to see some more painting mediums like pastel but maybe in the next version.
for now it is more like oil\acrylic\pencil.
there are ways to blend your colours after painting, but it feels more like dry brush.
something i didn't like much but i can live with it is that you can't go back to pencil after you start painting. But it's ok as it has a wide range of brushes round and flat.
overall i find it very handy, quite natural, and very comfortable and pleasant.
I am using a DSi XL where you have two screens i think a bit larger than iphone.
maybe on a regular DSi this might feel a bit smallish but still it will do the job.
It definitely replaced my pocket sketchbook!
you can save on your DSi photo album, you can save on the software itself, and you can save on SD as well.
the size of the images you see is the size that came on facebook (sent from the album of DSi XL) I have not yet saved on a SD so i do not know if this is the actual size of the saved images but i think it possibly is.
Shame I havent got a ds