Going in to the design process, I was ready to fire up KiCad. Then a full stop. The JLCPCB announcement page links to designing a PCB with JLCPCB's EasyEDA Pro software. Sigh, another program to learn. They have a version which can be downloaded onto a computer and a web application. I chose the web app and found it capable enough for my needs, although there seemed to be a few gotchas in the process.

Do refer to the guide linked above for the company details.

"At present, our production process exclusively supports Gerber files featuring colored silkscreen layers that have been exported from EasyEDA Pro."

There are selections in the EasyEDA Pro software allowing importation, use and exportation of color bitmap files, so their software is the only one to support this process at present.

Importing from Other PCB Programs

I did not try to design the basics of the board in KiCad (for example) and then attempt to import that into EasyEDA Pro for the PNG file. This approach might be possible.