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 helps an image to be displayed in appropriate resolution on any devices. It will work both with Bootstrap framework and without it. This checkbox being checked resizes image into smaller (preview) resolutions and adds the new srcset attricute to the image telling to the browser to get it from another URL when it is displayed on a device with small screen.
You can also use "Fit a container" checkbox together with "Responsive" one. 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 "Fit a container" is set.