User Story Thumbnail and Mobile Friendly Questions!

I am trying to complete the first FreeCodeCamp User Story challenge. I am having a hard time with the thickness of the thumbnail border…I don’t know why it is extending so far out from the picture. I also don’t understand the div class=col-xs-12 etc differences so i’m not sure how to make my code mobile-friendly. Any help would be much appreciated! Here is the link to my CodePen:

just add class="img-fluid" to your <img> html tag to make the image responsive