You can pretty much do this in the photo channel.
Create a 320x240 blank white image in whatever art program.
Make boxs of each color you want to act as your pallete.
Upload it to your SD card then to your Wii.
Goto doodle mode and viala.
Crappy paint program of pointlessness that everyone already knows about.
Post to your messege board to save as well.
Yep Im a smart ass :E
This however, is a good first step.
Nothing wrong with an art program to work with a pointing device.
Id love to see a pixel art program some day.
Maybe even a photo manipulator for taking heads off and putting them on random animals. (plus hats cant forget random silly hats)
Course once SNES9x for Wii gets super scope and mouse support we'll have the orioginal mario paint.
Now who needs photoshop when ya got mario paint? Eh ehhh?
Come on, you know you want it. xD