What version of Zencart are you running?
You need an HTML editor selected in Admin-My Store.
Check Admin-Configuration-Email Options that Use MIME HTML is set to true, and that Email Admin Format is set to HTML.
If you are using the Newsletter only subs mod, check Admin-Customers-Subscription Manager second column - Preference to see that you have a preponderance of customers with HTML selected. If not, you will have to check some individual customers. The default setting on the customer signup page is TEXT.
If you wish to use the admin email to test the newsletters, or still have problems, you should probably implement the changes mentioned here
http://www.zen-cart.com/forum/showthread.php?t=56507
If you wish to use FckEditor rather than HTMLEditor you will need to implement the change noted here
http://www.zen-cart.com/forum/showthread.php?t=58779