We use a simple html page for our emails, contains a small table, 1 image and the text.
We have used MS Outlook 98 to 2003 the same way.
Even with our spam filters, we can still send and recieve them and have not had any negative comments from our clients.
I think that you would start to get problems if you send out loads of mail at one time.
Have had a few problemd with McAfee anitspam detecting the image being sent if more than 4 mails are sent within about a 5 minute interval.
As for creating it, I use dreamweaver, but any HTML or text editor will do and use Outlook 2003.
So I can only explain for MS.
you need to save the HTML and image to the following:
\Program Files\Common Files\Microsoft Shared\Stationery
Then open Outlook and go Tools, Options, Mail Format (TAB), Click on Stationery Picker, then select the file you want.
Click OK etc when required.
You will then be able to open a new mail with the template you made.
Hope this help, or let us know.