Did you create in in File>Document Color Mode>RGB? The image should be RGB for web.
For a diagram with few colours, you may consider PNG or GIF for crisp appearance.
The dart diagram is not quite circular: it is slightly higher/narrower.
sorry for my lack of details but I have tried with both gif and png but the results it the same. The original Illustrator file is also in RGB colour mode. The problem occurs when I import the picture to the website (the file looks fine when it's on the webserver)
Which Edit>Color Settings profile do you use?
What happens if you use AdobeRGB (1998)?
What happens if you select Emulate Adobe Illustrator 6.0 or Color Management Off?
Have you tried turning off the Preserve option when Saving for Web?
My CS 2 is by default set to emmulating Illustrator 6.0 btu I tried to change for advence setup and choosed RGB1998 - still same results. On another computer I discovered the colour windows to have a CMYK background. Changing that to RGB (and hereby changing the colour slightly) I manage to improve the drawing and that's fine for now. Anyhow, thanks for your inspiration
Looking at the image again, I believe the (remaining) muddiness is caused by resampling of the JPEG image; resampling creates dirty specks and the like.
Are you sure the image dimensions have not been changed from original artwork over image creation to final web use, possibly twice? The image itself seems to be a good 1% from being circular (higher/narrower), and its appearance on the website seems to be almost 3% higher/narrower.