Some essential images were removed in the last pull request.

[#107](https://github.com/NeuronDance/DeepRL/pull/107)
[e637ba8](https://github.com/NeuronDance/DeepRL/pull/107/commits/e637ba8230189bd3927e35eb30e5836bf9af77df)
In this commit, lots of images(200+) in this library were removed and made some contents hard to understand. I'd like to ask for a revert. Is this because those images have copyright?
