Nothing!
Here's a few tips to make it easier. As long as you always center the object of focus within the inner third or centered 2/4ths of the image layout, you ought to be on solid footing no matter how the pre-fab settings drop, crop and lop the image depending on viewport.
Why is this? It's a simple calculation. Re-sizing of viewport affects elasticity of overall view by cropping out edges. Depending on the severity of the viewport resizing, this may result in as much as a 1/3rd reduction on every side. Dynamic re-arranging of modules is necessary to all responsive design.
As long as the subject matter you want to remain visible is centered within the inner 1/3rd or 2/4ths of your image, you will still be able to see it regardless of screen size.
