In order to have your image scale correctly on all viewports, you have to do the following. 

By default, it will work, but if for some reason, your images do not scale correctly on mobile, it is probably the height attribute that is set. 

If the height attribute is set, on a smaller viewport this will deform the image when making the image fit to a smaller screen, the height stays the same, thus scaling it nog right. 

Simply remove the height attribute from the image and your problem is solved.

see the steps

The image on the small screen with height attribute:

Check the code in the editor and remove the height attribute:

Check the result: