Could you please help me with my question about plant image classification?

Hello, I have a question and I don't know what should I do, I really appreciate any help with that. I have twenty images of twenty days of a plant and I need to cluster them, The pixels with high intensities are considered stressed and the pixels with low intensities are considered normal. I don't know how I should use clustering. I am not familiar with machine learning unfortunately but I need to do a class for a class project. Thanks so much for your help