const byte *spritesheet[] = { blank_tile,
bedtop, bedbot, bones11, bones12, bones13, bones21, bones22, bones23, bookstop, booksbot, ////////10
cabtop, cabbot, cactus, chair, chimneytop, chimneybot, coconut, couchtop, couchbot, debris, ////////20
This is not JSON data. This is an array of pointers. It looks like JSON data because Javascript borrows a lot of its syntax from Java, which borrows a lot of its syntax from C.
The issue on an arduino is that this data will be copied into ram at boot time. This is unnecessary, because it is static data. To fix this, you use progmem to tell the system not to do this. It does, however, mean that to get the actual data, you need to use the various built-in functions whose job it is to get data out of progmem.