I'm not sure I understand how to create the MouseOver and MouseDown JavaScript buttons. Here is a tips page for creating them. There is no way around this. The best thing in this case is to make the image either transparent or select the background color of the button so that it matches the webpage's page color. What happens is that only a single color can be designated as a transparent color, in this case white.
Whites then turn into shades of gray.
Gray is not set as the transparent color so those pixels will appear. Simply change the background color to closely match the web pages background color, regenerated the buttons and you should be good to go. Single transparent button with a white background as the base color. Single transparent button with a light gray background as the base color. Single transparent button with a dark gray background as the base color.