You will need to be able to reference the image somehow in your document. There are a few ways you could do this.
Access the temporary file from the uploads.
In your HTML file just reference the temporary file created by the upload as your image source, e.g.
<img src="<?php echo $_FILES['userfile']['tmp_name']; ?>">
Capture the content of the image and write it into the HTML as a data-uri value
dompdf understands data-uris and so you could take the uploaded file, convert it to a data-uri, and insert that into your HTML.
You do have to detect the image format, which can require a bit more work depending on your version of PHP. With v5.3+ you can use the finfo_file method. If that extension isn't available you can perform some basic content-type detection by parsing the file extension.
In my example I'm just specifying PNG as the image type.
<?php
// detect mime type, but I'm forcing PNG
$mime = 'image/png';
?>
<img src="<?php echo 'data:' , $mime , ';base64,' , base64_encode( file_get_contents( $_FILES['userfile']['tmp_name'] ) ); ?>">