You will have to include the complete URLs in the source for the newsletter.

quick tip - for using full URLs, if you are using the FCK Editor in the newsletter module to write your email, go the

FCK Editor 'Show Custom Editor Options'
Expand 'Other Editor Options'

Tick 'Full image path'

At the bottom - 'Apply Custom Settings to - in the drop down list select 'instance' and it will just apply those settings to when you are making edits on that page.

Click on Apply, Close the window

Then click on Refresh Editor link

Try adding an image and see if you get the full URL created

Thanks,