MSPaint seems to make a dogs breakfast of saving anything other than bitmaps, though. The saved image quality with some file formats is pretty shabby, I've found.
Because each image that is created or loaded in, MSPaint tries to save a bit of memory, by defaulting to save it as the lowest quality it can. To get around this, and preserve quality, always Save As 24-bit image, then load back in and save as Jpg or whatever.