I've attached a patch which implements displaying remote images.

> This is a longstanding problem, and there was an attempt to patch it in
> 2014, but the patch was never accepted:
> https://lists.gnu.org/archive/html/emacs-orgmode/2014-11/msg00583.html

Compared to the previous attempt from 2014, I think my patch is simpler
-- it doesn't require creating any temp files.

> The fault might be with image.el rather than with org-mode itself --
> for example, when I execute the following elisp, I get the same blank
> box:

After doing some reading, I learned that image.el doesn't really create
the image. Instead, create-image simply creates a blank string with a
text property pointing to the image file location, and the rendering of
the image gets handled later by the C code (for example, png_load_body()
in image.c), which doesn't know how to read remote image files.

Since I wasn't comfortable trying to get the C code to read the remote
file, I instead took the approach used by image-mode.el, which reads the
remote image file and passes its contents directly to create-image,
instead of just passing the filename.