How DOMPDF translates those paths depend on how the file was loaded.
If you use the load_html() method DOMPDF will assume all paths are
relative to the PHP file. If your PHP file is in a different location
from your HTML document then the paths will need to be different.
Additionally, absolute paths (e.g. "/pics/some_pics.gif") are assumed
to be off the root of the file system, not the root of the web
content.
You can try setting the base path by calling $dompdf->set_base_path().