Are we talking about Rose Marie Reid nee Yancey or Rosemarie Bowe? One was a swimsuit model and designer while the other was an actress. Life did do a 2 1/2 page spread on Reid nee Yancey's designs. From that, I discovered:
https://www.google.com/search?q=Rose+Marie+Yancey++Life+Magazine++1956&client=firefox -b&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjvpqm4yNnbAhXk24MKHRbECK4QsAQIJg&biw=16 00&bih=729#imgrc=x2jistYyaYCbaM: which took me to pinterest and
https://www.oldlifemagazines.com/the-1950s/1956.html where you can buy old issues of life (which might be your best bet to finding it).
Also found
https://books.google.ca/books/about/LIFE.html?id=sUEEAAAAMBAJ&redir_esc=y - which allows you to browse the old issues. It might take you a while but you could find it there.
But I also found something that said "July 1956 Vogue magazine featuring the Rose Marie Reid Ad" - which means it might be Vogue.
Hope this helps! And if it does, post a link to whatever photo you find.