The cover girl for the Dizzy album is Jane Asher, who was Paul McCartney's girlfriend at the time. She appears on the cover with her back to the camera, standing at the entrance to a tunnel.
However, there are a few things to note:
* There are different versions of the cover. The original UK pressing features Jane Asher, while the US version, and some later UK pressings, feature a different woman, believed to be a model named Susan Leach.
* The album cover was actually meant to be a still from a film. The Beatles were working on a film called "A Hard Day's Night" at the time, and the tunnel scene was supposed to be a part of it. However, it ended up being cut from the film and used for the album cover instead.
So, to answer your question directly: The cover girl for the Dizzy album is Jane Asher, but only on the original UK pressing.