As a scoundrel who commonly uses pics from other people's sites I learned to add the 'alt=' to every one - because that picture remains dependent on the original website for generation. Too often I run across an old post where I didn't add a descriptive label and see the broken pic icon; making me wonder what the picture was.
The 'alt=' short description remains even if the pic's original domain was wiped out, so whatever point one was trying to make with the picture isn't lost as time moves on.
Example:
I might have 'borrowed' a humorous depiction of James Carville. But if the people in control of the image's source take it down I still have:
ReptilianCarville
instead of the
originally posted. It's a consolation prize that insures your efforts weren't wasted due to someone else's actions.
But the RIGHT thing to do is to post pictures from your own computer doubling as a server OR using a service as the posters described above.