A sentence that includes imagery and repetition is the following:

On went her old brown jacket; on went her old brown hat.

This sentence repeats the words "old brown" and includes the imagery of the old, drab-colored clothing that Della must wear because she and her husband are so poor. The repetition of the words "old brown" emphasizes how depressive and worn-out her clothing and her apartment are. The "old brown" of Della's hair is in contrast with the "brown cascade" of her lovely hair. While her hair is also brown, like her clothes, her hair is glorious, almost to the point of being a luxury. Therefore, her decision to cut off and sell her hair is the greatest sacrifice she can make, as her long, brown hair is her only possession of any value. The "old brown" jacket and hat that she has to wear highlight the brilliance of her hair. 

The line that stands out the most to me in terms of the imagery it presents through repetition comes after Della has finished crying, and she takes out a rag to dry her cheeks.  

She stood by the window and looked out dully at a gray cat walking a gray fence in a gray backyard.

Della is quite, quite down as a result of her meager savings. She really wants to buy her husband, whom she loves very much, a lovely gift for Christmas, and she simply cannot afford it. In this line, she is described as peering out the window dully, and this idea of dullness is echoed by the repeated use of the word gray. Gray is such a dull color, never bright nor cheerful, but always muted. The repetition of the word gray—three times in this sentence, in fact—certainly works to describe the way she sees everything in her life in this moment. Everything, for her, feels gray: dull and muted. She isn't happy, and this sentence conveys that quite clearly.

The story begins with what becomes a melancholy refrain. Della would like to buy her husband a nice Christmas present, but she only has one dollar and eighty-seven cents.

ONE DOLLAR AND eighty-seven cents. That was all. And sixty cents of it was in pennies.

The narrator repeats the amount of Della's meager savings three more times.

One dollar and eighty-seven cents. And the next day would be Christmas.

Tomorrow would be Christmas Day, and she had only $1.87 with which to buy Jim a present. 

Only $1.87 to buy a present for Jim. 

The reader can not only visualize the sum of $1.87 in small coins but can also picture Della counting them over and over again, as if she were hoping as if by magic they would amount to a little bit more. This would seem to be a very good way to represent the whole setting and reflect Della's mood. O. Henry must have thought the same, because he repeats more or less the same words four times. The reader understands not only Della's sorrow but her strong motivation to get Jim a really good Christmas present in order to show her love and to give him some enjoyment in life after a long year of working for a miserable twenty dollars a week. Della keeps counting her money because of her motivation, and her motivation will make it credible that she should reach the sudden drastic decision to sell her beautiful long hair.