Saving Images in Paint
For this one, I will be using a faerie uni image, and going from worst to best.

Monochrome Bitmap=A BIG BIG NO NO. You don't even need to ask what's wrong with this picture.

16 Color Bitmap=Same as above

Gif= It spreads little dots all over and makes your pic look a bit fuzzy.

Jpeg= Fuzzes up your image a bit

24 Bit Bitmap= Same as above.

PNG= Changes slightly, but best way to save in paint.