I wrote a small extension for the Python-Markdown library which adds Bootstrap 4.0 styling to images and tables generated by the generator, available on GitHub.
Call this extension towards the end of your extension stack; it simply adds
<img> tags and
class='table table-striped' to
<table> tags. This extension probably isn’t life-changing and it is probably also under-tested (in particular, there is no support for Python 2.7 which I don’t use and don’t intend to support). However, it goes a long way towards making images and tables behave nicely in sites using Bootstrap + Python-Markdown, both by respecting responsive layouts and by just looking nicer.