1
general::
Phrase(s): *pretty as a picture
very pretty. (*Also: as ~.) • Sweet little Mary is as pretty as a picture. • Their new house is pretty as a picture.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
Very attractive, as in She looked pretty as a picture in her new hat. The noun picture alone was used to describe beautiful objects from the early 1800s on; this locution, however, dates from about 1900.
American Heritage Idioms