OK, two questions:
1) Is the HTML you are sending to these recipients static, (ie. apart from the recipient email address everything is the same - although that's not really a requirement, it could be built on the fly) ?
2) Does it have to go through Outlook, (I mean, do you
REALLY have to use Outlook to send it) ?
If the first answer is Yes and the second No then there is a very simple way to achieve what you want using a simple command line program.
SendEmailCreate your HTML message as a file embedding the graphics as Base64 encoded text and use it as an input to the program:
eg.
<html>
<body>
This is your basic test email with embedded clickable image.<br>
<br>
<a href=http://www.donationcoder.com/forum><img src="data:image/jpg;base64,iVBORw0KGgoAAAANSUhEUgAAAIAAAACACAMAAAD04JH5AAAABGdBTUEAAK/INwWK6QAAAwBQTFRF
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</body>
</html>
Then in your command file you just need the line:
sendEmail -f fromemail@fromserver.com -t recipient@recipientserver.com -s smtp.fromserver.com -u "Invitation" -o message-file=Invitation.html -o message-content-type=html
I've skipped the options for TLS, user, password, etc but they can be added if your mail server requires them.
I've tested this through Google' email system and it works fine.
The requirement to use Outlook shouldn't really be necessary as there will be a record of the sent email on the server that can always be accessed via IMAP/etc.
I sent you a test email to show you what you'll get

Here's an online
Base64 encoder for converting the images.