Mary Gernat was very well known for her illustrations for Armada books in the 1960s, when their books had cover illustrations, rather than photographs.  She illustrated many, many books for them in her inimitable style, which is not one that appeals to all.  To me, her illustrations have energy and excitement, but to others they are awkward and anatomically suspect!

She did covers for many pony books, but only two Jills:  the Armada 1963 printings of A Stable for Jill, and Jill Has Two Ponies.  Both retained the original internal illustrations by Caney.  

Jill keeps her pigtails on the cover of Stable, but looks a rather different girl on the cover of Two Ponies.  Rapide and Black Boy (though thankfully the right colours) look more like horses than ponies.