That is her on the cover
Yennifer looks like shes in her late 20s whereas Ciri at the time was still a 15 year old tomboy. In the final book, the knight (I forget his name) thought she was so beautiful that she had to be a fairy or some other supernatural being. Meanwhile, she was just bathing in a lake.
In the novel she is concidered extremely beautiful with several people trying to sleep with her per book. After she received the scar on her face, she was concidered beautiful but disfigured.