There should be several file formats for graphics.
In my personal opinion, MAG is one of the easier to use and viewer.
It is possible to use those 640x400 4bit color images on MSX.
Those images are displayed as 512x424 4bit images on Screen7.
BLS could viewer TXT,PIC,MAG,MAX.

PIC is fine for the 512x512 16 bit images of Sharp X68000.
This is a Japanese format.
When you use this file on MSX2/TurboR the image is downgraded to 256x424 8
bit interlaced image.

BLS also feature a Screen 12 to Screen 8 filter!
8-bit images are cool!!!