Images of a pony sitting or standing on all fours should be a minimum of 800px in height, head-to-hoof. 1200px is preferred. Images of ponies standing on their hind legs or in an unusual pose should be proportional.Note that this refers to the resolution of the character itself - not the canvas.
The pony in this image is only around 320 pixels tall, way below the minimum requirement.
The rules don’t say anything about the shadow, but I think it’s probably not allowed, because it’ll cover other ponies.