What is the best way to store images in python for machine learning
This might be a bit more complicated.
I normally reuse computer vision and deep learning software to do that. Even if I don't do Deep Learning.
Particularly I use Pytorch, for its bridge with Numpy and pandas.  Here is a tutorial.
This allows me to use a GPU if wanted, and to reuse a lot of code since for deep learning and images there is tons of code snippets out there.