How can I make images in a markdown block alternate wrapping left and right?

I am trying to wrap my text around my images. I have two images and I want one to wrap on the left and the other to wrap on the right. Here is my code:

    .markdown-block .img:nth-of-type(2n){
       float:left; 
       margin-right: 20px;}
    .markdown-block img:nth-of-type(2n+1){  
       float:right; 
       margin-left: 20px;}

However, this is resulting in both images being wrapped on the right side.