UPDATE 2013-09-25
Kerin Portillo Reddit AMA
Found her, the Sheltered College Freshman on I-710 in Long Beach. She will wash your truck clean.
The stock photo used on the billboard can be found at dreamstime.com, titled Bikini Woman Smiling.
Her picture on the billboard is also from the same set featured at Know Your Meme: Sheltered College Freshman
cReddit: Kerin Portillo