Here's something from a newsletter I sent myself:

And that's the test. The images don't appear (with or without a table). Any ideas why?

OK, a little deeper digging here found the answer,

The newsletter editor doesn't form the FQDN for the graphic file it inserts; only the relative name.

Once you add http:// and the domain name as a prefix, the picture gets delivered.

What's "obvious" is what you're born knowing, the rest you have to learn.

Well, I'll just jump in and toss in what worked for me to see if that helps you out as I was having the same sort of trouble mentioned here some time ago and worked it out. 

For one site that I handle, it is an early version of DNN4.x and so it used the standard text editor that DNN has used for some time.  With that editor I could never get the images to come through the email newsletter even when going into the html view of the newsletter and adding the full domain name of the site before the image location.  What I ended up having to do for this site is download and install the FCKEditor Provider and configure its setup in the config file as the instructions included with it walk you through doing.  With that in place, when I would select the image button and choose an image to add, on the link line I would instert in front of the image location the full domain name and that took care of the problem.

For another site that I handle, which uses the current release of DNN, it is already preconfigured to use the FCKEditor Provider and so with it's newsletter all I have to do is just put the full domain name in front of the image location and it works.

Hope this helps you out.