How to make image 100% wide on mobile phone?

I have a simple html with 2 columns in a table. The left column is with an image, the right column with text. Now the column with text should shift under the image if the width < 550. The shifting works nicely but I would like the image not to stay small but go to 100% width on mobile phone. I added a code for this in the css (img[class=“img-max”]), but it does not work. Could anyone help to get it working?

The code is in this jsfiddle: https://jsfiddle.net/Kippie32/2hqg7xzs/