Pawns can't be on the first or last rank (they'd have to be promoted). There are plenty of impossible (illegal) positions like double mate, and more complicated

Pawns can't be on the first or last rank (they'd have to be promoted). There are plenty of impossible (illegal) positions like double mate, and more complicated ones where for example there are two dark-colored bishops + 8 not promoted pawns. Perhaps that kind of info will come in handy when the image algo is in doubt.