Hi Infininja. I'd say "go for it" about getting ArtAcademy. It's been well worth it to me. I'm still draw/painting on it since I downloaded them at Christmas. To answer your questions:
Saving and exporting is easy. You can hit save at any time while you're drawing. You can close ArtAcademy and come back to your drawing at any time. However, once you save another drawing you can't go back and re-edit an older one.
To export them there is another save feature that sends them to the DSi camera's photo album. At that point you can add a frame to your pictures if you wish, or just export them without. From the photo album you can upload them to a SD card, or send them to Facebook if you have an account. I sometimes save onto the photo album, then view on the computer so I can zoom in to see if there are any spots that need touching up. Then I go back and re-work them on the game.
Yes, I believe all the pictures are 480 x 640.
There isn't an 'undo' function. Like painting for real, once the paint is down, it's down. Though the pencil feature does have an eraser.
You can't really work on more than one picture at a time. However, if you download both semesters of Art Academy you can have a picture going on each one. I'd recommend doing that.
I wouldn't consider myself an artist by any means, but I've have a ton of fun drawing with Art Academy. Here's a link to my work if you'd like to see it. http://vatalbot.deviantart.com/gallery/
