you want to make a shape behind the image you're working with, and color it the same color as the background of your web page. what's happening is that illustrator is making a transparent image and it starts to act funky. if you're using a white background, then it should be fine.
it's kind of hard to articulate, so i've attached a quick example.
ok. on the left apple, when i created the graphic, i put a black box behind it then i saved it for web. on the right apple, i just made the object and saved it for web. the pseudo transperancy just screws it all up.
i'm not a web designer, but I hope this helps.