That was the whole point!
I think I found it. I should just replace .png with -2x.png in the welcome message. Then I had to tweak the sizes of some images. Now it works OK for me.
The English version of the copy has the correct widths and heights in the markup; use those. Copy the whole a href over.
The problem arises with the customized messages that rely on the old images:
https://meta.discourse.org/t/all-images-disappeared-in-my-customized-welcome-message/31253
It is obvious customized messages cannot be updated automatically, so why would you make the images unavailable at some point in time.