Some images just compress better than others. Read up on Information Theory if you're keen to understand why
![Smilie](/vbiis/images/smilies/happy19.gif)
Images with lots of fine detail will suffer most.
If you have an image that compresses badly you can always downsize it a little further before compressing it. That will give you a smaller image but with less obvious compression artifacts.
The ultimate solution is to include a link to a high res version of your image on an external site as well as the horribly compressed attachment.
Cheers,
Rick.