Thanks so much.
Let me explain.
The images I want to classify are not the natural images. They are spectrograms, the x-axis corresponds to time, the y-axis corresponds to different frequencies. To classify this kinds of image I think the CNN is not appropriate, cause the spectrograms are not natural images like pictures of cats or dogs.
So I want to use RNN, specifically, the LSTM, to read the content of each image according to the time sequence (the arrow direction).