The long answer: It depends on what your biggest constraint is. If your biggest constraint is server cost, then I would suggest C++. C++ has come a long way in terms of web services, and if you are doing both image processing and machine learning, then C++ will give you unparalleled performance and access to libraries. However, if your primary constraint is developer hours, then python is the way to go. You won't have the same explicit control, but for OpenCV and Machine Learning, Python has excellent support and you will end up having much more concise, readable code.
It runs on both osx and linux for me
There is also bindings for OpenCV here is one