For your question about manipulating images when moving from desktop to mobile or vice-versa,
There is many methods but I used to do this one:
you have to add the two images (desktop & mobile) to your html code and set the mobile image display property to hidden . and when you move from desktop to mobile by media queries you have to set its display property to block (or inline) and for desktop image set display property to hidden. By this you will be able to change between desktop and mobile images.