I got a message from Ville Jouppi stating that the original Kickstart screen was created by Sheryl Knowles. Seeing that her initials are S.A.K. we figured that she must also have been the artist behind the SAK images.
At the same time there was Jack Haeger with the Commodore team and with a little imagination the ljl Signature can look like his initials J.H.
I renamed SAK and LJL to the proper names and also regrabbed the Amiga Boing Ball Demo image. While looking at it closer I found out that it was put together pretty cleverly.