Can't embed image in PDF letter

When using the "Print PDF letter for Contacts" action, I tried to add an image that I uploaded via the WYSWYG. When I downloaded the PDF file, in place of the image, it said:

DOMPDF_ENABLE_REMOTE is set to FALSE

It's a reasonable default for the library, since it prevents malicious users from inserting untrusted content. However, since we expect only trusted users to be generating PDFs it seems unnecessary. Furthermore, many groups don't have printed letterhead. It's much cheaper to upload their letterhead/logo image to their PDF letters directly and then print.