The issue has been fixed in Zimbra Desktop 2 Beta 3 which is due to be released tomorrow (07.06.2010
Zimbra Product Portal )
If you don't wish to install that, you can save these images from the email via slightly long and unwieldy workarounds.
The related bugs are here
https://bugzilla.zimbra.com/show_bug.cgi?id=35956 and here
https://bugzilla.zimbra.com/show_bug.cgi?id=46245 .
Either:
a) access Zimbra Desktop through a webrowser, and then try and right-click the image to get a 'save image as...' option.
NB To use your default browser to display Zimbra Desktop UI, go to the account setup page and click on "open in web browser" link at the bottom.
[You may run into this bug:
https://bugzilla.zimbra.com/show_bug.cgi?id=28495 you need to start the printing process, and you should be able to open the image from the print preview screen as I understand it.]
OR:
b) open the email in Zimbra Desktop normally, click and drag the areas before and after the image to highlight it, use CTRL+C to copy the image (and whatever else you've highlighted) and paste it into a graphics or wordprocessor document (such as GIMP or openoffice.org Writer).
Good luck.