I note too that neither ":image" nor "image spec" has a substring match
for `i'.  (Yes, I know that you will reply that the former cannot be
indexed - dommage.)

FWIW, these (attached) are the only index entries I see when matching
"image" as a substring.  Dunno where to find the info about "image
specifications."  (Even searching for ":image" did not seem to help.)