pretty as a picture
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.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.