You can be guided by this example. On your css file:
.deskContent {
background-image: url(../img/big-pic.png);
width: 100%;
height: 400px;
background-repeat: no-repeat;
background-size: contain;
}
.phoneContent {
background-image: url(../img/small-pic.png);
width: 100%;
height: 100px;
background-repeat: no-repeat;
background-size: contain;
}
@media all and (max-width: 959px) {
.deskContent {display:block;}
.phoneContent {display:none;}
}
@media all and (max-width: 479px) {
.deskContent {display:none;}
.phoneContent {display:block;}
}
On your html file:
<div class="deskContent">Content for desktop</div>
<div class="phoneContent">Content for mobile</div>