What means "Responsive" checkbox on the image?
I see the checkbox "Responsive" on the left sidebar when I click on the image. Looks like it changes nothing, could you explain what does it mean?
Responsiveness is the main feature of Bootstrap framework. You see this checkbox because you use Bootstrap Editor together with N1ED. This checkbox being checked adds special class to the image telling to it to change its size (to shrink) when it is displayed inside block with small size.
You can see this effect when switching to preview another breakpoint (i. e. "XS") on the ruler above the document. It will turn your editor to mobile simulation mode and probably make your blocks smaller. If you image is big enough it will shrink to fit its parent block in case checkbox "Responsive" is set.