My first landing page

I have three questions:

• I dont know how to set the menu beak's responsive in all devices.
• To set the responsives images, is it better to set a div with background-image and background-size: cover, or set an img element with srcset of images in different resolutions??
• In the second case, the project only has 2 images resolutions, so the media queries will be mobile and desktop ones, would be better have tablet images resolutions too?