I'd like the carousel to scale proportionally the background image

in this question https://stackoverflow.com/questions/11734195/bootstrap-carousel-image-does-not-scale-proportionately it is suggested to leave the image explicit dimension but I can't understand what it means.

Set this in your HTML

For every img element in your carousel, you can set the width property to 100% like this (although you shouldn't need to if the natural resolution is larger than the control):

<!-- language: lang-html -->

<img src="http://i.imgur.com/OEdBxVD.jpg" width='100%'>

• normally, this image would be too small to expand fully, but 100% scales it up.
• make sure you don't have the width or height set to any pixel sizes

jFiddle

Since the image is going to go full screen, you should try to get your hands on a resolution that will naturally try to take up the full width available. When the browser has to scale the image up, it will cause artifacts.