This is a seemingly unpublished, and perhaps one-off illustration for A Voyage to Arcturus. Titled "Chapter IX — The ride on the shrowk", it is by Hugh Deane, a Kensington-based illustrator who worked for periodicals of the 1920s & 30s. It is remarkable for being an illustration to David Lindsay's novel done close to the period in which the book was published.
Many thanks to Roy Sinclair for providing this.
See also the Wormwood blog for some information about Deane, and this illustration.
If anyone has any additional information about this illustration, do please contact me (via the links page).