There is actually no mention of a rose anywhere in the story, and that is part of what makes the title of the story all the more intriguing and significant. In order to come up with an idea about the rose reference in the title you must first brainstorm what you know about roses and what you associate with roses. Your list could include some of the following ideas:
1. Roses are beautiful, but deceptive, as they have thorns.
2. We associate roses with love -- white ones for a wedding, red to express true love, etc.
3. We use roses as a flower of tribute -- think of weddings, funerals, pageant winners, actresses after a performance, even the winner of the Kentucky Derby horse race is given a wreath of roses to wear around its neck.
With all that in mind, you need to then connect those ideas to the character of Miss Emily. The narrator of the story tells a shocking story of Miss Emily and her keeping of the murdered body of Homer Barron in her upstairs room, but he also spends a good deal of the story explaining what kind of person Miss Emily was and how she was treated in her life. He ends up creating a character that we could have some sympathy for. While no one would say it is OK that she killed a man so that he wouldn't leave her, the narrator explains how her father drove away all of suitors and how the town treats Emily with a mix of pity and annoyance through the years. This story then is a tribute of sorts to Miss Emily. The narrator is careful to give us a more sympathetic and developed characterization of a sad and pathetic woman, and that is a kind of tribute to her, rather than merely presenting the shocking gruesomeness of her actions with and against Homer.