Xylography is the art of engraving what?

Xylography is the art of engraving on wood, or of printing from such engravings. The word xylo is a combining form meaning “wood”, and the word graphy is a combining form denoting a process or form of drawing or writing.

Woodcut is a relief printing technique in printmaking. An artist carves an image into the surface of a block of wood—typically with gouges—leaving the printing parts level with the surface while removing the non-printing parts. Areas that the artist cuts away carry no ink, while characters or images at surface level carry the ink to produce the print.

The art of carving the woodcut can be called "xylography", but this is rarely used in English for images alone, although that and "xylographic" are used in connection with block books, which are small books containing text and images in the same block.