We record whether pictures or sounds have been used before by storing single byte flags in the following array. (The extra 5 values allow for the fact that it can be legal to use low undeclared sound effect resource numbers in the Z-machine for short beeps, though this is deprecated in I7.)
Pictures and sounds are identified within blorb archives by resource ID numbers which count upwards from 1 in order of creation, but can mix pictures and sounds freely. (For instance, 1 might be a picture, 2 and 3 sound effects, then 4 a picture again, etc.) ID number 1 is in fact always a picture: it means the cover art, and is the I6 representation of the value "figure of cover".