As I can't attach zip files, I will paste in the minimal project I've used
intro.md:
Welcome
====
Image is out of bounds ...

The template is a basic one with two master entries: Break and Content. The only customisation is the page size: Format -> Page and set Width to 10.00 " and Height to 5.63 " .
The image I've used is https://watirmelon.files.wordpress.com/2012/01/automatedtestingpyramid.png .
Hopefully that is enough to reproduce the issue