Some examples of prepositional phrases functioning as adjective phrases would be the following:
Amy showed me a picture of her new puppy. (“picture” = noun being described; “of” = preposition; “of her new puppy” = adjective phrase)
Have you read the Shakespeare play about a Scottish king? (“play” = noun being described; “about” = preposition; “about a Scottish king” = adjective phrase)
The tall girl from my English class is on the basketball team. (“girl” = noun being described; “from” = preposition; “from my English class” = adjective phrase)
My uncle with the lake house invited us on summer vacation. (“uncle” = noun being described; “with”= preposition; “with the lake house” = adjective phrase)