The preferred way of using images is placing them in the /assets/images/ directory and referencing them with an absolute path. Prepending the filename with {{ site.url }}{{ site.baseurl }}/assets/images/ will make sure your images display properly in feeds and such.
Standard image with no width modifier classes applied.
HTML:
<img src="{{ site.url }}{{ site.baseurl }}/assets/images/filename.jpg" alt="">
or Kramdown:
Image that fills page content container by adding the .full class with:
HTML:
<img src="{{ site.url }}{{ site.baseurl }}/assets/images/filename.jpg" alt="" class="full">
or Kramdown:
{: .full}
