Rosie appears in the music store when Eddie Kerns sings his "Broadway Melody." She is the one in the black dress with the deep white "V" in front, with several swingy pearl necklaces. She is to the left of Eddie and dances while he sings.
The actress who plays Rosie doesn't even get a credit--and she had at least one speaking line. What a bummer! She was the epitome of the 1929 Flapper Girl!
It's too bad that IMDb doesn't have her credited in the cast list, so we don't really know who she was. But she had lines so she wasn't just a bit player.
I really liked the look of all the women in this movie. The chorus line dancer with the headache stands out as a counter-example to the stereotype of the flat-chested flapper type. That girl was stacked!
You might have deduced from the dialogue that Rosie and her cohort were apparently ready to do a menage a trois with Eddie to get their hands on his song. They are both very cute and "full of pep" (as the slang at the time went), but they could not hold a candle to Queenie (Anita Page).