Great feature, sometimes we should fetch the preview in case our server has no permission to get the image.
I think first try "upload from URL" and only try local upload (automatically) as a second try is a better choice.
After our server failed to fetch the image, we could upload it via browser-file.

Any further changes?
I prefer to try downloading image on server before we upload from local file, haha.
It seems that current implementation is "upload local file directly".

Yeah, I have to work on it still. Probably will have a bit of time today/tomorrow.

Expecting your great work!
I've add new checks on front-end building workflow, don't forget to rebase to master to apply the change : )

I tried updating this code again, but there are CORS problems too unfortunately:

The resource at “https://instagram.fmxp6-1.fna.fbcdn.net/…” was blocked due to its Cross-Origin-Resource-Policy header (or lack thereof). See https://developer.mozilla.org/docs/Web/HTTP/Cross-Origin_Resource_Policy_(CORP)#

These problems can be avoided in the browser plugin, but this needs a major change in cooperation between the bookmarklet (which alone cannot read images from cache) and the browser extension.