When using Theatre Manager's WYSIWYG option, you will need to do a one-time setup to connect to the Apache server 'htdocs' folder. Then just create your email blast letter inside Theatre Manager.
For the option of designing web pages externally, an excellent tool is Dreamweaver. If you have your non-profit id, this can be purchased for $25 at Tech Soup. Its handy for editing things and viewing web pages.
Or, your web designer, can post the page on the internet and you can view it with internet explorer, firefox, safari of other browsers. Or they could email the HTML and if you save the file on the desktop, just drag it onto your web browser to see it.
This this, every venue has the software to view an html email it on every workstation in the office.
Before sending, make sure the images are visible.... if they are, then: