Large and small images need different treatment for optimal results.
Large images (over 300px wide)
On mobile devices large images should scale down to fit on small screens.
Always check the responsive display checkbox:
Small images (less than 300px wide)
Small images such as logos and social media icons should stay small; they should not scale up to take up the full width of small screens otherwise they break your layout:
Small images need to be resized using the re-size function. It's not enough to resize them using the width/height fields.
One your images are resized to be less than 300px wide, clear the responsive display checkbox to prevent these small images from scaling up.