I'm sure there is a simpler way to do this, but here is how I found a way to add an image into the body.
1. Open up zimbra and compose a new message
2. With the new message open, click the options tab near the top (right above the To: section" and choose Format as HTML
3. Once in HTML, click the Add Attachment button and attach the image you want to use.
4. Once it is attached you will see a link to the file under the subject. Right click on the image and choose to open it in a new window.
5. When the new browser opens with just the picture, click on it and drag it into the body section of the email.
6. This will add the image into the body section. You can then uncheck the attached file under Subject.
Again I'm sure there must be an easier way, but hopefully that will work for you.