The subject matter of American Gothic, 1942 is a Black Woman maid standing in front of the American Flag. The main characteristics of the photograph are The Rule of Thirds, Contrast of Light, and Dark and Patterns. We see the rule of thirds characteristics in the sense that both the broom and the mop are on the lower left and right thirds respectively. The subject and the tools of her trade are also in the lower third of the photo. There are also the stars on the flag in the background which serve as a pattern. One of the most obvious parts of the composition is the contrast between light and dark considering the subject is lit from the left side, the right side of the subject is in darkness, giving a dramatic look.
This photo was taken in 1942 in Washington D.C. To understand this photo you need to realize this photo was taken in 1942, at a time where Black Americans weren’t treated equally and segregation was very much still a pressing issue. The subject of the photo is charwoman Ella Watson. After experiencing Washington D.C. in 1942 without his camera as a status symbol and being treated just as horrible as any Black man in America at the time, he sought the story of Ella Watch who allowed him to take her photo and opened up her home to him. The fact that Parks, composed this photo with a Black woman with cleaning supplies in front of the American flag at a time where Black people were not treated fairly in America conveys the idea that while Black Americans continued to help build and maintain the country that is the USA but wasn’t given the same right as everyone else.
You are on to something in your last statement when you state that Parks wanted to convey how Black Americans helped build and maintain the US without fair treatment but you also left something out.
In the resources I supplied, it covers that Parks was quoting a famous painting in this photograph called American Gothic. The original shows a white farmer and his daughter as the salt of the earth, the bedrock of the country. Parks by quoting the painting is saying Black Americans built and maintain the country, are its bedrock. It’s in the quotation of this painting that Parks makes his point.