Dompdf is optimized for HTML rendering, but if for some reason that's not feasible you can also add an image to the PDF directly using the canvas object.
With the releases available at this time (latest release being 0.8.0) you should be able to do this using something like the following code snippet:
$dompdf = new Dompdf();
$dompdf->loadHtml($html);
$dompdf->render();
$canvas = $dompdf->getCanvas();
$canvas->page_script('
$pdf->set_opacity(.5);
$pdf->image("/path/to/image.png", {x}, {y}, {w}, {h});
');
Where:
- the image is on the local file system
{x}
= horizontal (X) position from top left
{y}
= vertical (Y) position from top left
{w}
= image width
{h}
= image height
- opacity is a decimal value from 0 (transparent) to 1 (opaque)