What to input into machine learning algorithm for image recogniton?
Usually, neural networks are used to do image classification. The reason is that image data is very complex (highly non-linear) if you want to say so. 

Have a look at Keras/Tensorflow. You can even use pre-trained models to quickly achieve good results on most problems. Find an example for Python here: https://github.com/Bixi81/Python-ml/blob/master/keras_pretrained_imagerec_binaryclass.py

There is a very instructive book „Deep learning with Python“, which can give you a quick start. Find the book here: https://github.com/fchollet/deep-learning-with-python-notebooks