Unfortunately, no free images of Rosa Parks exist for download. Furthermore, only a small number of paid stock image sites host images of Parks. Most images of Rosa Parks are held by private individuals and companies, who must be contacted directly for permission.
One of the least expensive ways to acquire a copy of a Rosa Parks stock photo is through the website Dreamstime. This website has several stock photos of Rosa Parks, and as of March 2015, its cheapest subscription is $0.05 per week, which allows four image downloads. Alternatively, many university libraries have copies of Rosa Park photographs and may allow you to use their images for free, so long as they're credited properly.