Whatever default thing your scanner does. It doesn't matter if its default JPG, many Brawl screenshots are JPGs. However, it's highly suggested that if it doesn't save in JPG as its default extension, PNG works better since it's a bigger, less compressed file format with no noise or blur and it can be saved in transparency.
PNG: Most game screenshots, sprites, artwork etc.
JPG: Some game screenshots (Brawl specifically since bin2jpg produces high quality JPG without noise and blur), most photographs, some huge artwork
If it's a big enough image the jpeg compression won't hurt the quality too bad, but if you have the hard drive space (and you really should in this day and age) PNG is preferable for pretty much anything.