All files must have a width that is a multiple of 4. This is because the BMP encoding pads all rows to a 32 bit size.
It would be nice to be able to change the blocksize. But doing the thing described below doesn't seem to work. Probably the implementation of LittleFS does not support this. I don't know.
To allow for more efficient use of the storage change the blocksize in:
boards.txt (~/.arduino15/packages/esp8266/hardware/esp8266/3.0.2/boards.txt):
d1_mini.menu.eesz.4M3M.build.spiffs_blocksize=4096
Maybe also this file eagle.flash.4m.ld (~/.arduino15/packages/esp8266/hardware/esp8266/3.0.2/tools/sdk/ld/eagle.flash.4m.ld):
PROVIDE ( _SPIFFS_start = 0x40308000 ); PROVIDE ( _SPIFFS_block = 0x1000 );