It can read a path to a local image, but not to a remote image.
If PHP would be able to read data from a remote host, all computers would be hacked in no time. As far as I know, you can't get any access to remote files on a users computer, unles he specifically allows you too (through a 'Open' dialog window, or by opening a port, ....) via a web browser (or for that: any server-sided programming language).
The reason why you could read the image on your local machine is because the fonts are located on your local machine itself.