Some images just compress better than others. Read up on Information Theory if you're keen to understand why
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.