For my Jekyll blog, I want the images to span the whole width of the column, while having padding on either side of the text, like this: http://www.webdesignerdepot.com/2015/05/
I understand that you are writing your post/page in markdown.
In order to apply a specific style to the P container you can use kramdown block attributes to set a class on it.
P
Some test ![Alt text](/path/to/img.jpg) {: .imgContainer}
Will render as
Some test
You can then style .imgContainer.
.imgContainer