The pictures in header or sections, you usually call images, are actually CSS backgrounds. Those background images can not be responsive as standard images but can respond to resizing and scaling.
CSS backround on desktop
CSS backround on mobile
To resize background on smaller screens and not just to cut it, you need to set COVER background position. This option is available for each background image and is located next to that option.
If for some reason, you decide no to display background images in sections on mobile devices, we have an option for that purpose too. To display mentioned backgrounds on desktop only, please go to Betheme > Theme Options > Responsive > Header > Options section and select Show on Desktop only for Section | Background Image option.