Solution: Embed the Image(s) in the HTML markup.
While I still have not gotten as far as testing it end to end, it does work as far as putting the image in the proper place in the PDF Preview screen.
In my case I did not even try using the provided form to insert an image because I was pretty certain I wanted more control over where it was placed.
So what works for me is the following in the HTML markup:
<div class=”page-container”>
<div class=”page-header” style=”margin:0;”>
<div style=”float: left;width: 20%;”> </div>
<div style=”float: left;width: 80%;text-align: center;font-size: 25px;font-weight: 700; margin-top: 50px;”><p> <br>EMS Road Docs of Illinois<br>RELEASE and WAIVER of LIABILITY</p></div>
</div>
</div>
I still have some tweaking to do with that, as this plug-in does not appear to recognize some of the CSS I have above even though the documentation for mPDF suggests that it is indeed supported.