It's a printer's term that comes from typography. There, images were on separate blocks, either all-metal or more often, metal mounted on wood to be level with type.
EDIT:Like this:
https://goscf.com/t/51921Perhaps since images were usually ganged up for etching or casting and the result sliced into individual parts for mounting on wood blocks (see the nails above), that's where the word comes from. Just a guess.