Contrast means the juxtaposition of two subjects that are very different. High versus low. Old versus new/young. Black versus white. One colour versus its complementary colour. Rough versus smooth. Hot versus cold. Big versus small. Bright versus dark. Rich versus poor. And so on. As a theme for a contest, “Contrast” does not mean artificially boosting up the « contrast » function in Photoshop/Lightroom/etc.