2-D mel frequency cepstrum conversion

I am a computer engineering freshman. I am in a project where I basically try to classify pneumonia positive and pneumonia negative using CNN. I alreday got the initial result but I am tring to improve the performance by converting the pneumonia image into mel frequency cepstrum. However,Matlab function mfcc only can convert audio signal. I just had intro to signal processing class, so my understanding in frequency domain is very limited. I would really appreciate it if anybody can help.thanks!