Your pony is too small (reminder that you can, in fact, use sizes other than the predetermined screen resolutions). You’ll have to get her larger. No, adding blank space won’t work.
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.
Sorry, i will try to resize it sometime, or ask somebody to draw it.