Bootstrap is a great option to consider. Specifically, you should take a look at
.img-responsive
-> This class ensures that an image is responsive and scales nicely to fit the parent element.
If you want more information, check out this link: http://www.w3schools.com/bootstrap/bootstrap_ref_css_images.asp
Once you've explored that, delve into using rows and columns with bootstrap as well. It really simplifies everything in my opinion.
Regarding the border issue:
border-top: 1px solid white;
border-bottom: 1px solid gray;
I trust this will be of assistance!