I use a program called "Colors!" it saves everything automatically as a .bmp (I think) and has a really nice interface. I hope that helps.

Oh, thanks! BMP - it is very good.
What about max resolution (size of picture in pixels?).

And, there have "zoom" function?

The picture is the size of the DS screen in pixels. I think it changes if you use the zoom function though, which yes, is there. What happens when you use the zoom function (at least, this happened to me when I used it) is that the picture becomes as large as how many times you zoom in. Basically, if you zoom in 3 times (which I think is the maximum) the picture is 3x the size of the DS screen. I'm not entirely sure though. There is a website for it here: http://colors.collectingsmiles.com/downloads.php

Happy drawing!

Thanks a lot for exclusive info!