I’ve just run into the same question (pat on the back for searching the forum first!).
I am assuming that I want to select “custom Image” when I’ve connected to the ESP through the FluidNC Web Installer? [ Connect>Upgrade FluidNC Button>Install a custom image ]
The installer tells me to select the firmware.bin file but I’m not certain where that is in the package I downloaded from @Michael_Melancon’s post above. I feel like I may be doing this wrong. This board was recently reflashed and the configs loaded following the instructions but I do believe I was able to reflash just using the dropdown versions available in the Web Installer. Not sure how to do what I have to do here.