Custom certificate (plugin)

I would like to add my signature to a certificate, but it seems that putting it on /pix/signatures would render it available to all courses, not just mine. This is on an university-wide server, so I don't like it

I see that in the custom text is allowed < img > (src and width (or height) are mandatory)
But where should be the image to be loaded with src?

Note that I am not the administrator

thanks
Piersante