OK, in case somebody wants to do the same thing than me, and assuming here a square aspect ratio, this CSS will ensure that both "contain" background images and percent sprites are always visible and stay at the same place, whatever the screensize is :)
#tuesday {
aspect-ratio: 1 / 1;
height:auto !important;
max-width: min(100vw,100vh);
margin: 0 auto;
}